I definitely would not require a feat for this... a donkey's CR is 1/6, which falls into the 'below average' range of challenge ratings for animals that can be used as familiars.
Tome and Blood's improved familiar feat allows use of things like Psuedodragons as familiars. It doesn't quite fit:
"What can I take as a familiar with this feat?"
"Well, you can have a shocker lizard, a homunculus, a psuedodragon... or a donkey."
As for kicking the heck out of goblins, kick isn't listed as an attack for a MM donkey so I would only use the listed bite... perhaps for added realism the DM could just tell the player "Your donkey informs you that it
could kick the goblin, but feels that this would be undignified. He suggests
you try kicking the goblin instead." Hey, they're supposed to be stubborn!
Besides, anyone who would be willing to send such a nice, big, targetable familiar into battle probably deserves the penalties from losing their familiar, or getting told off by an ass.
For all the abuse the player is going to take for having a strong spiritual bond with his ass, I really wouldn't penalize him. I might even provide a nice bonus for the caster. Carry capacity for a familiar is unusual, but the limited use of a donkey familiar (how many inns will let you bring a donkey inside, how noticable is a donkey running alone through the forest scouting ahead, and so forth) without spell enhancement more than balances this in my mind.
As to the original question, you could play a donkey familiar in my campaigns any day, it's worth it for entertainment value alone.
It would also provide an excuse to whip out the Shrek DVD, so we can do 'research'.
