In Harry Potter and the Prisoner of Azkaban, the name of chapter 17 is Cat, Rat, and Dog. Peter Pettigrew is the rat and Sirius Black is the dog (their respective Animagus forms). But who is the cat? Prof. McGonagall wasn't there, so it couldn't have been her in her cat Animagus form.
Who is the cat in chapter 17 of Prisoner of Azkaban?