Crookshanks is a fictional character from the Harry Potter Universe.
Crookshanks is Hermione's cat that is half-cat, half-kneazle. He is quite intellegent and very wary of suspicious characters. Hermione bought him at the Magical Menagerie store in Diagon Alley. Crookshanks was the first person to realize that Scabbers wasn't actually a rat, but instead was Peter Pettigrew in his Animagus form.
He was in the Magical Margarine for a long time but no body wanted him till Hermoine came along. Ron wasn't too happy about the cat.