Croockshanks

Crookshanks is the pet cat of Hermione Granger. "Crookshanks" means, "bent legs"; in keeping with this, cat is described as bow-legged when the characters first meet him. He was described as having a "squashed face," which was inspired by a real cat J. K. Rowling once saw that she said looked like it had run face first into a brick wall. Hermione bought Crookshanks from a shop in Diagon Alley out of sympathy, as nobody wanted him due to his squashed-looking face.

Crookshanks had a habit of trying to hunt down the pet rat of Ron Weasley, Scabbers. This caused a great deal of fighting between Ron and Hermione in the third book. Rowling has confirmed that Crookshanks is half kneazle, an intelligent, cat-like creature who can detect when they are around untrustworthy people, explaining his higher than normal cat intelligence and stature. Because of this, he was immediately aware that Scabbers, Ron Weasley's pet rat, was not a real rat, and that the huge black dog lurking around the school was not a real dog. Crookshanks was proved right when it was revealed that Scabbers was in fact Peter Pettigrew, whereas the dog was Sirius Black. Sirius eventually persuaded Crookshanks to trust him and sent him to bring Pettigrew to him; Crookshanks, who had been pouncing on Scabbers from the moment the two had met, evidently agreed. Afterwards, Crookshanks played no major role.

It had been suggested that Crookshanks is an Animagus; however, J. K. Rowling has officially confirmed that he is not.
(Source : http://en.wikipedia.org/wiki/Magical_creatures_(Harry_Potter)#Crookshanks)