• Custom item of master cavalier^CC: For 18,000gp, get an untyped +20 bonus on Ride checks for your special mount or similar companion (or +10 for other mounts). Less than half the price of a +20 competence bonus item, and stacks with it.
  
• Gloves of the Balanced Hand^MIC: This 8,000gp item grants Two-Weapon Fighting as a bonus feat, or Improved Two-Weapon Fighting if you already have it. Two-Weapon Fighting is a prereq for the Ranger Knight and Revenant Blade PrCs.
  
• Humillianthir^DrComp: Two suits of +3 leather armor (one of them barding) which function as a set - if a paladin wears one and his special mount wears the other, then he can greater teleport onto his mount from up to 500ft away as a move action. The total cost is 3,000gp more than two normal suits of +3 armor.
  
• Rider's shield^RoS: This exotic shield's AC bonus also applies to your mount. Use when you would normally use a heavy shield. You can take the Exotic Shield Proficiency feat to use it, or trade in your existing Tower Shield Proficiency.
  

• Hound archon: A hound archon with access to a special mount can choose to use a bronze dragon, which gains an additional +2 HD, +4 Str, +4 natural armor, +10ft speed and improved evasion (...despite special mounts already having improved evasion). This does not provide you with the dragon, so you'll have to find one yourself (such as via Dragon Cohort or a fellow party member) but it doesn't suffer the normal +2 LA for cohort mounts. Also hound archons can turn into dire wolves while bronze dragons can turn into Medium humanoids, which means you can reverse the roles if you want. Hound archons themselves suffer from their huge +5 LA; take a level in Ranger Knight and see if you can convince your DM to treat their racial HD as levels in a divine casting class for the Holy Mount feat. Or play a gestalt campaign maybe.
  

• Barbarian (Horselord^Dr338 ACF): Replaces rage with "Battle Ecstasy", which grants different bonuses and doesn't affect your concentration. You also get a ranger animal companion, some skill bonuses, and Improved Mounted Archery as a bonus feat. Take 6 levels in this class at most, unless you get something awesome from another ACF later on.
  
• Bard (Fey bard^UA ACF): Trade bardic music and knowledge for druid animal companion and some other minor abilities.
  
• Bloodlines^UA: Not technically a class, but close enough. Levels in a bloodline advance every class-level-dependent ability you have at once, including animal companions, familiars and special mounts; in some cases (e.g. Celebrant of Sharess 10) this can even allow you to break the cap and start using the epic advancement rules. Titan Bloodline lets you wield a Gargantuan warhammer in one hand (which is probably just poor wording, but cool enough that some DMs allow it anyway). Celestial Bloodline grants Smite Evil, which lets you qualify for Devoted Tracker if you gained a special mount from an unusual source. All major bloodlines grant 2 fixed bonus feats, which are occasionally useful for prereqs.
  
• Cleric (Shaman^Dr353 ACF): Trade turn undead for a ranger animal companion. It's normally painful to give up access to Divine and Devotion feats, but a mounted character won't get much benefit out of Travel Devotion anyway.
  
• Cleric (Champion^UA ACF): Trade turn undead for smite evil + aura of courage. Useful mainly for meeting the prereqs of Devoted Tracker.
  
• Druid (Focused Animal^Dr347 ACF): Lose trackless step, woodland stride and wild shape(!). In exchange you get +2 to your effective level for your companion's abilities. If you were taking 4 or fewer levels in druid then you won't feel the loss, otherwise ouch. As a side bonus you can speak with animals at will, which is great for roleplaying interactions with your mount.
  
• Druid (Urban companion ^City ACF): Gain a slightly-improved familiar in place of an animal companion; among other things its hp are 3/4 of yours rather than half.
  
• Monk (Holy monk^Dr310 variant): Trade your lv1/6 bonus feats for Aura of Courage and Smite Evil, plus paladin Turn Undead at lv4. Another way of meeting the prereqs for Devoted Tracker.
  
• Monk (Shinto monk^Dr358 fighting style): At 6th level, gain an animal companion as a druid of 1/3 your monk level. Given the bonus feats provided by this style, consider on a Saint and/or Cha-based build.
  
• Ranger (Fangshields^CoV RSL): +2 to your effective druid level for determining your animal companion's abilities (making your companion equivalent to a druid's at the level you get it). Cannot be selected by members of the Humanoid type (the Otherworldly^PGtF feat comes in handy here).
  
• Ranger (Planar ranger^UA ACF): Your animal companion gains the celestial or fiendish template for free.
  
• Ranger (Urban companion ^City ACF): See the druid version of the ACF above; your effective druid level is still halved for determining your familiar's abilities, but you get more benefit from the hp thing than a druid does.
  
• Scout (Light cavalry^Dr346 ACF): Gain a paladin special mount at 5th level, except that it can't share spells or command creatures. Your fast movement and free movement class features apply to your mount instead of you; you don't get camouflage or hide in plain sight, but both of you gain trackless step. If you're using this ACF, pick up smite evil somehow (a paladin dip is the easiest way but there are others); that way you can qualify for both Swift Hunter and Devoted Tracker.
  
• Sorcerer (Blood of Eberron^DR351 ACF): Replace familiar with ranger animal companion, Natural Spell feat works for any kind of shapeshifting.
  
• Wizard (Animal companion^UA ACF): Gain a ranger animal companion in place of a familiar. Sorcerers can select this option too, but Blood of Eberron provides more benefits.
  
• Wizard (Enchanter variant: Cohort^UA): Trade your familiar for a cohort, as the Leadership feat (does not stack with the Leadership feat). Note that these specialist wizard variants are banned by many DMs.
  
• Wizard (lv5 "High One Warrior-Wizard" substitution level): Combine your familiar with your special mount, as long as you have at least 2 levels of paladin. Interestingly, if you traded your familiar away for another ability then you gain one for the purposes of this effect, so you might as well take the animal companion ACF above then combine all three with Devoted Tracker.
  

• Aglarondian Griffonrider^UE: Get a griffin companion with its own progression OR select a griffin as your special mount and advance it. Note that granting your mount Evasion doesn't do anything in the latter case, since it already has Improved Evasion. You can also use Mounted Combat multiple times per round. The capstone works like Spirited Charge, but stacks with it. Sadly incompatible with special cohort mounts.
  
• Celebrant of Sharess^PGtF: An exalted PrC which can be entered at lv5 rather than lv6. Grants a cat familiar as a wizard/sorcerer of double your class level (doesn't stack with wizard/sorcerer levels, but should stack with PrCs which advance familiars explicitly), as well as some random stuff like rage, pounce, quasi-bardic-music and limited spellcasting. It's not much by itself (the chassis is pretty bad), but it has potential in combos.
  
• Faith Scion^UA: If you're using the rules for legendary weapons, this class will advance a ton of abilities at once. Oh, and you also get a hammer.
  
• Halfling Outrider^CW: Dual advancement for animal companions and special mounts.
  
• Human Paragon^UA: Lv1 lets you add one skill as a class skill to all classes. Picking up Ride or Handle Animal like this can be very useful for some builds.
  
• Knight of the Iron Glacier^Frost: Advances your special mount and replaces it with a "war megaloceros", a megaloceros (CR4 Animal with Improved Grab, Scent and a mini-dungeoncrasher ability) with the warbeast template already applied. This is basically a 1-level dip class.
  
• Ranger Knight^Dr317: Grants an interesting collection of abilities, most notably a special mount at +5 effective level, favored enemies, and upgrading Trample to a full attack (devastating for dragon mounts). You can also treat medium armor as light armor for the purposes of all armor-restricted abilities while mounted. While there's four prereq feats, it's pretty easy to gain them through ranger levels and magic items (such as battle bridle and gloves of the balanced hand).
  
• Silverwood Outrider^Web: Grants benefits for Wild Cohorts, animal companions or special mounts. However, since it does not actually advance their abilities, there is little reason to use this for an animal companion. With a special mount it makes even less sense, since special mounts are intelligent and don't need to be handled or learn tricks. A one-level dip makes Ride a class skill for all classes, which even applies retroactively - don't take any more levels than this unless you have a Wild Cohort.
  
• Vadalis Beastkeeper^Dragonmarked: Get the magebred template for your animal companion or special mount, some mounted combat bonus feats, and limited spellcasting (including polymorph and a version of heal mount which also works on animal companions). What's more, you can enter this PrC at lv5 rather than lv6 like most PrCs. A good choice for use with the Holy Mount feat in Ubermount builds.
  
• Wild Plains Outrider: Advances either animal companion or special mount
  

• Aereni Focus^PGtE: 1st-level-only version of Skill Focus for elves, which also makes that skill a class skill for all classes (and counts as Skill Focus for prereqs). Useful if your classes don't normally have access to skills like Ride or Handle Animal.
  
• Celestial Mount^BoED: Special mount gains celestial template. For an ubermount with huge numbers of HD this can be worth adding for the Spell Resistance, but normally the bonuses aren't really significant. Note that it requires 4 levels of paladin, meaning prestige paladin can't be used for the usual 2-level dip.
  
• Cloak Dance^XPH: Take this if you find that you have move actions to spare while mounted. You gain concealment as a move action, or total concealment as a full-round action.
  
• Close Cohort^DR346: If you have Leadership your cohort is one level lower than you, rather than two levels. It isn't much of a stretch to apply this to alternatives like Dragon Cohort. Talk to your DM before trying to combine it with thrallherd.
  
• Co-ordinated Strike^RotW: +1 attack bonus against any foe your animal companion or special mount has struck in the same round. I'd just take Knowledge Devotion instead. Heck, Weapon Focus is better than this.
  
• Despana School^DotU: Grants bonuses when fighting alongside creatures you've summoned (including auto-flank), making this neat for paladins, knight phantoms and so on. You need summon monster to qualify, which means either an arcane casting dip or ex-paladin/blackguard.
  
• Devoted Tracker: Lets you designate your special mount as your animal companion. The possibilities are endless. Forms the basis of the ubermount build when combined with a way to advance both at once (such as bloodline levels or certain PrCs).
  
• Dragon Cohort^Drac: Like Leadership, but better. Well, if you wanted a dragon anyway.
  
• Dragon Steed:
  
• Exalted Companion^BoED: Adds some good-aligned magical beasts to your list of potential "animal" companions. Note that this lets your companion qualify for exalted feats without combining it with a special mount.
  
• Fast Rider^DR285: Your mount gets +10ft speed when it has no less than a light load.
  
• Feral Companion^CoR: If you are evil, your animal companion gets +2 Str and Con. Also its bite is diseased and it's harder for others to handle, but those are little more than fluff.
  
• Gestalt Anchor^CP: +2 to Ref and initiative is pretty nice but your mount needs to take a [Host] feat, and you need to either do the same or be a kalashtar. Choose the latter if possible. See also Psymbiot.
  
• Githyanki Dragonrider^MM4: A githyanki with this feat gains +2 Diplomacy/Ride vs red dragons, and both it and its mount gain +1 AC/Reflex.
  
• Holy Mount^DR325: All levels in divine spellcasting classes count as paladin levels for determining your special mount's abilities. Paladin's a pretty weak class compared to most divine casters, so this means you don't have to take many levels in it. Gets better with cleric X/prestige paladin 2 builds. Nasty when combined with Devoted Tracker.
  
• Improved Familiar:
  
• Improved Psicrystal: Ignore the extra personality. What you're interested in is the +1 level for determining abilities, which means +1HD, which means extra attack bonus and feats for your psicrystal. Basis of the "Big guy is with me" build.
  
• Leadership: Source of mount with its own gp supply.
  
• Lifebond^LM: An undead creature who takes this feat chooses one living creature, and gains +2 saves/+4 turn resistance while it's within 60ft (or penalties for 24 hours if it dies). Could also be useful in the other direction, for an undead mount ridden by a living character.
  
• Mounted Combat: The basic riding feat, and a prereq for most others. It's pretty easy to get a higher Ride check than your mount's AC, so it's a useful effect.
  
• Mounted Fighting^Dr346: +1 attack/damage while mounted. Better than Co-ordinated Strike, but still very weak (particularly since you can't take it at low levels).
  
• Mounted Fury^Web: If you can rage, your mount can rage with you. The benefits are obvious. Note that you must be riding your mount when you start raging, and the mount stops raging if you dismount.
  
• Natural Bond: Practiced Spellcaster for animal companions - increase your level by 3 for the purposes of determining its abilities, to a maximum of your character level. It's a reasonable stretch to allow this to apply to Wild Cohorts as well. This gets interesting when applied to beastmasters, who have a level cap 3 higher than normal. It gets really interesting if you can use it to get higher-level companions faster.
  
• Obtain Familiar:
  
• Phalanx Fighting^CW: Consider this if your mount can hold and use a shield (via race or feats). See "feats for mounts" for more details.
  
• Psicrystal Affinity: Source of potentially impressive mount, but only if transformed.
  
• Psymbiot^CP: Grants benefits if both you and your mount have manifester levels of 3. The latter can be handled by having your mount take a combination of Hidden Talent and Practiced Manifester, or a [Host] feat, which also qualifies you for Gestalt Anchor.
  
• Saddleback: 1st level only. You can always take 10 on Ride checks. Once per round you can reroll a Reflex save as a Ride check - this applies to either you or your mount, or both if you must both save against the same effect. Your mount probably has Improved Evasion, so the second one is more useful when used on yourself. Still, on a clumsy character the ability to reroll Reflex saves at a higher bonus is hardly shabby, and taking 10 is useful too.
  
• Share Soulmeld^MoI: If you can already shape soulmelds, chances are good that at least one of them will benefit your mount.
  
• Shared Fury^RotW: Basically the same as Mounted Fury, but for an animal companion within 5ft.
  
• Slow^UA: Both the trait and the flaw of this name reduce your speed. A dwarf PC or one wearing heavy armour who takes both will find himself with a speed of only 5ft per round. However, as long as you stay on a mount your own speed will never come up, meaning that you essentially get Improved Toughness and a second feat for free.
  
• Spider Companion^DotU: Adds monstrous spiders of various sizes to your list of animal companions.
  
• Strength of the Charger^OA: Odd feat. Bonus to hp and Fort saves while mounted equal to mount's Con modifier.
  
• Talenta Dinosaur Bond^PGtE: Grants +4 Ride/+1 AC/+1 Reflex to a rider, and +1 AC/+1 Reflex to their mount... as long as you're a halfling riding a dinosaur. Pity it doesn't count as Skill Focus for prereqs.
  
• Theurgic Bond^DR325: Allows levels in animal companion classes to stack for familiar abilities or vice versa. The "vice versa" is the more interesting here, since familiars don't get nearly as much as animal companions.
  
• Theurgic Mount^DR325: Arcane version of Holy Mount. Probably weaker unless you have combined your mount with your familiar. The Fey bard variant which receives an animal companion can use this to great effect with Devoted Tracker.
  
• Thrall Bred^LoM: Choose one creature with more HD than you. You gain a +1 bonus to saving throws and damage rolls while they're within 30ft and doing okay, and a -2 penalty when they're not. Ubermounts will easily have more HD than their riders, so why not? Doesn't work the other way around since it's Humanoid only.
  
• Totem Companion^ECS: Adds a specific magical beast to your list of animal companions.
  
• Tunnel Riding^RoS: Your mount does not take the -4 attack/AC penalties for squeezing through Medium passages. The alternatives to this are having your mount take the Tunnel Fighting feat, or being a Small character with a Medium mount.
  
• Underfoot Combat^RotW: When sharing the space of a creature two sizes or more larger than you, you gain soft cover (+4 AC). For Small riders with Large+ mounts (or Medium riders with Huge+ mounts, Tiny riders with Medium+ mounts, etc.).
  
• Vermin Companion^ECS: Adds some vermin to your list of animal companions. Unlike standard vermin they have Int 1.
  
• Wedded to the Light^Dr358: Grants the familiar of a Good character a few extra abilities; among other things it can sacrifice itself to resurrect you, and you both get fast healing. Can be selected as a wizard bonus feat.
  
• Wild Cohort^Web: Animal companion on the cheap, with entry to two interesting PrCs.
  
• Winter's Mount^FB: Special mount gains cold subtype and +2 Con. If your mount already has the fire subtype this will make it immune to both cold and fire. Otherwise, the fire vulnerability offsets the extra HP somewhat.

• Lance: While mounted, you can wield a lance in one hand and your charge attacks deal double damage. Note that just because you can, you don't have to wield it in one hand. Most charger builds benefit more from a two-handed weapon and Power Attack
  
• Lancer's Spurs^CC: If you wear these boots while riding your special mount, the mount gains the benefits of an amulet of mighty fists +2 (at half the price!) and their natural attacks become good/evil-aligned depending on your alignment. If you ride a mount that's not your special amount, the bonus is +1 and there's no alignment effect. Since this replicates the war-mount spell in the same book, see if you can convince your DM to let it trigger the spell's synergistic effect with master cavalier and phantom charge.
  
• Battle Bridle^MIC: 9k, grants Mounted Combat and a +5 bonus to Ride checks. If you already have Mounted Combat you get Ride-by Attack instead. A melee character should upgrade to Riding Boots when it becomes possible.
  
• Riding Boots^MIC: For 12,000gp you get +5 to Ride checks, Ride-by Attack as a bonus feat, and a boost to the power of Spirited Charge if you have it.

• Bladebearer of the Valenar^RoE/PGtE: Elf-only feat that's functionally equivalent to Exotic Weapon Proficiency (double scimitar), but you also get to share feats like Weapon Focus between scimitars, falchions and double scimitars, and they deal +1 damage while mounted. There's another minor bonus which applies if you're using action points. But most significantly, this is a prereq for Revenant Blade (see below).
  
• Cavalry Charger^CW:
  
• Ride-by Attack: While not as good as Fly-by Attack, this is much easier to qualify for than Spring Attack, and you get more total movement out of it (a charge as opposed to a move action). Note that you must continue in a straight line after you attack, unless you or your mount has a "crooked charge" ability of some kind. The downside is that, well, by RAW it isn't actually possible to keep moving in a straight line after charging except in a small number of situations, since this would result in you moving through your enemy's space. The designers probably intended either that you could end a charge in any space from which you could attack the target, or that you could make an overrun attempt as a free action as part of using this feat (it seems to have been written before those rules were finalised), but make sure you clear up this issue with your DM.
  
• Spirited Charge: Double damage on mounted charge, or triple damage if you use a lance? Boo-yah!
  
• Spirit of the Stallion^PGtE: +1 on melee attack and damage rolls, but only if you're an elf riding a Valenar horse (which is a specific creature, not a fluff thing). It's better than Weapon Focus, but there are much better things you could be spending feats on.
  
• Talenta Warrior^RoE: Like Bladebearer of the Valenar but for halflings; the weapons are Talenta sharrash, Talenta tangat, and Talenta boomerang.
  
• Trample: This feat was clearly written with horses in mind - it's not clear what happens when something like an ashworm is entitled to a free "hoof attack", though it should probably be taken to mean their primary attack. If you can use this feat with a mount with a nasty primary attack then this can be decent, particularly if you have access to Fell Trample. Usually, though, it's better to just have your mount trip the enemy while you attack it. Consider having your mount take Improved Trip instead.
  
• Valenar Trample^PGtE: A variant of Trample which lets both you and your mount attack, and counts as Trample for prerequisites. Nice. However, it has the same restrictions on use as Spirit of the Stallion.
  

• Bone Knight^5N: Grants a special mount which stacks with paladin levels, and applies the skeleton template to it. Also you get armour made out of bones.
  
• Cavalier^CW: This class takes your Spirited Charge feat and turns it up to eleven. Unfortunately, you can only use its insane damage multipliers a limited number of times per day.
  
• Crinti Shadow Marauder^SS: A 5-level PrC which grants Sudden Strike +2d6, and lets you and your mount teleport short distances or shadow walk. The last level gives you access to the amazing Shadow Pounce ability, which lets you make a melee full attack as a free action after teleporting, as long as your target was in line of sight beforehand. Could be a nice combo with the phantom charge^CC spell.
  
• Knight Phantom^5N: Gish class - 9/10 spellcasting, full BAB. Your phantom steed advances faster (+10CL!) and you also get it as a 1/level spell-like ability. Importantly, the SLA version takes only a standard action to cast rather than the normal 10 minutes. You also get some phantom-themed abilities.
  
• Revenant Blade^PGtE: If you have ranger levels, levels in this class stack for your animal companion and caster level (the wording is very specific). Also you get three floating feats from a short list, and can treat both ends of a double scimitar as two-handed weapons. You must be an elf, which limits things somewhat (look into wood elf or wild elf for more melee-friendly ability modifiers).

• Steadying Robe^CM: You no longer have to make Concentration checks to cast spells while being jostled around. And for only 1,000gp? Very nice. By item-stacking rules you could make this slotless for 2,000gp, in case you're a gish who doesn't want a robe taking up his armour slot. Otherwise, most robes can be crafted as cloaks without an increase in price.
  

• Mounted Casting^MiniHB: You gain a +10 bonus on Concentration checks made to cast spells while mounted. Nice, but pointless if you can get Mounted Spellcasting instead. Due to the wording, however, you might be able to interpret this as a bonus to all spellcasting-related Concentration checks (including casting spells while taking damage), as long as you are on a horse while you make them.
  
• Mounted Spellcasting^Web: Superior to Mounted Casting, this feat lets you cast spells while mounted without having to make Concentration checks for movement. Requires 5 ranks in Concentration though, so you'll have to wait until 2nd level.
  
• Spiderfriend Magic^DotU: Your spells can't harm Vermin allies. Even if your mount is a Vermin you might not need this, but if you do use spells which could harm your mount it's great. Ignore the "you can suppress this feat for 1 round as a swift action" part - you can already suppress it just by not considering someone an ally.
  

• Alienist^CArc: Summoning-focused PrC which advances your familiar and grants it the pseudonatural template after 5 levels (don't get too excited; it's the Complete Arcane one, not the Epic Level Handbook one).
  
• Arcane Hierophant^RotW: Dual-progression arcane/druid, which advances your familiar+animal companion and combines them (if you don't have a familiar then you gain one for this purpose; effective level = levels in arcane spellcasting classes + AH levels). You can enter as a non-druid via the spirit folk (bamboo)^UA race or a wildrunner^RotW dip, which meets the Trackless Step prereq (Cleric/Fey bard could be an interesting option).
  
• Fleshwarper^LoM: 9/10 spellcasting PrC which advances your familiar, makes it an aberration and grants it four bonuses from a list; Size Increase in particular is powerful if your familiar is unusually large already.
  
• Knight Phantom^5N: Gish class - see melee section.
  
• Nar Demonbinder^UE: Summoning/binding-based PrC with unusual spellcasting advancement (similar to sublime chord). Grants a familiar from your wizard/sorcerer levels plus your Nar Demonbinder levels, and it can be an imp or quasit as if you had the Improved Familiar feat (which, in alternate form, are suitable for use as mounts by Small creatures).
  
• Silverwood Arcanist^Web: Requires Wild Cohort. Grants faster handling, Mounted Spellcasting as a bonus feat, and Share Spells. Full spellcasting advancement.
  
• Talontar Blightlord^UE: 8/10 casting PrC that applies the blightspawned template to your animal companion at lv1, making it a Plant with Str +4/Con +2/Wis -2/Cha -2 plus the standard immunities.
  
• Wizard (martial wizard^UA): Select your bonus feats from the fighter list instead of the wizard list; Scribe Scroll is also replaced by a fighter bonus feat. A useful ACF for meeting prerequisites.
  
• Zhentarim Skymage^LoD: *snorts milk out nose* This would be powerful even without the mount it gives you... which can be basically anything. Oh, and you can share spells both ways, meaning that you can ride a dragon and have him share his buffs with you.
  

• Master Cavalier^CC (Blackguard 2, Clr 2, Pal 2): You gain +10 on Ride checks for 10 minutes/level, or +20 for your special mount "or similar class ability".
  
• Phantom Charge^CC (Blackguard 4, Pal 4): As a swift action, you and your special mount or fiendish companion teleport 5ft/2 levels; the spell can explicitly be cast in the middle of your mount's move. If you're under the effect of master cavalier and war-mount, your mount also gains a fly speed equal to its land speed (average maneuverability) until the duration of one of those spells expires.
  
• Spirit Steed (Deathless domain^ECS 4, Siyal druid 4): For 1 hour/level, gives an animal +30ft speed, Run as a bonus feat (stacks with the existing feat if they have it), and no consequences for hustling (effectively doubling its speed over long distances). Afterwards it becomes fatigued.
  
• War-mount^CC (Blackguard 3, Pal 3): Grant your special mount or fiendish companion a +2 enhancement bonus on its natural attacks, and they strike as good/evil weapons depending on your alignment.
  
• When Two Become One^OA (shugenja [water] 3): Basically a low-level version of the fusion power for riders and mounts.
  

• Supermount! (original thread)

LG Ranger 4/Cleric 1/Prestige paladin 2/Vadalis beastmaster 10/Beastmaster 3
Feats: 1: Mounted Combat, 3: Skill Focus (Handle Animal), 6: Devoted Tracker, 9: Natural Bond, 12: Dragon Cohort, 15: Holy Mount

At lv20 your companion is any dragon available as a cohort with a net -1LA reduction, +20HD, +24 natural armor, +10 Str, +6 Dex, +10ft speed, Empathic Link, Multiattack, Improved Evasion, Share Spells, Share Saving Throws and Spell Resistance. On top of that it gets +4/+2 to two physical ability scores, your choice of +2 natural armor or +10ft speed, and Alertness, Athletic, Endurance or Improved Natural Attack as a bonus feat.

Elf Paladin 2/Wizard (High One warrior-wizard, UA animal companion, martial wizard) 6/Ranger knight 1/Fleshwarper 10/Wizard +1
Feats: 1: Aereni Focus (Ride), 3: Track, 6: Natural Bond, 9: Graft Flesh, 12: Devoted Tracker, 15: Theurgic Mount, 18: ?
Flaws: Any two
Bonus feats (flaws): Mounted Combat, Trample
Bonus feats (wizard): Two-Weapon Fighting

Your companion is a Special mount 23//Familiar 17//Animal companion 6, resulting in +12HD, +23 natural armor, +6 Str, +2 Dex, +10ft speed, Empathic Link, Multiattack, Improved Evasion, Share Spells, Share Saving Throws and Spell Resistance, plus four Aberrant Familiar abilities (Size Increase can be very powerful on something that's already big). You have a free feat slot for Dragon Cohort if you want. Also you cast spells as a 16th-level wizard.

If you drop the Graft Flesh feat and replace the Fleshwarper levels with wizard levels, then you gain access to 9th-level spells and your mount gains an additional +2 to HD/natural armor/Str/Dex.

You can get rid of the reliance on flaws by using items (such as Battle Bridle and Gloves of the Balanced Hand) to meet prerequisites.