I got some nice feedback from Bernice, who said:
If he stayed as a donkey, decided he was happier being a donkey and just eating grass and carrots and being petted, than being a wizard and teaching monstrous children/being a spy, joined a donkey sanctuary, ran away, went out to stud, and studded a little baby donkey on a jill, I bet it would be a donkey centaur.
Yes, I am insane to even consider that...
And when Snape is finally forced to return home to Hogwarts, as a wizard, he spends the next 20 years being followed around by a small, greasy, grey furred, black haired donkaur.
And then, she drew a picture! When I asked for permission to put it up on this site, she said:
Yes, certainly. If you put it up with your story, you can either:
a) put up my explanation, or, probably better
b) write a short note that ties it in to your story. Say... I don't know, Mpreg style. Sirius gets Snape pregnant... while in donkey form... oh, that makes my heart hurt. And Snape then has to stay a donkey for however long donkeys gestate, because he's... an intrasexual donkey? I feel a migraine coming on, but then this is what he gives birth to.
c) there is no c).
d) of course, he adores his donkaur.
With that kind of buildup, I'm sure you're just on pins and needles waiting for the picture: