hm. it doesn't say so in the 图书 或者 movies, and his just disappears without forming completely. jo herself hasn't revealed it either. but i would expect it to be a badger, toad, tiger, 或者 some other animal that represents him. ~potternerd~
I know for a fact he can produce a corporeal Patronus. 你 see it in OOTP. It bursts from his wand, making him stagger, and trips Ron. From what I could see and hear, it is a canid of some kind.
