The changeling wizard substitution levels from Races of Eberron let you shift your familiar between all its possible forms. There was talk about the cheesiness of combining this with Improved Familiar. However, you can go further.
Paladin 5/Wizard 5
High One Warrior-Wizard racial substitution level 5 (Champions of Valour Web Enhancement) lets you treat your special mount as a familiar. While it opens up more options, this isn't the trick. According to the DMG, a paladin with the Leadership feat may treat a cohort as his special mount for +2 LA.
That's right, your familiar can now turn into any character four levels below you at will. You can even turn it into another wizard and have it share spellbooks with you (though that would be a waste, since it has any spell you want, ready to cast).