No, Crookshanks was not an Animagus. However, he was a very clever cat and had great abilities for a normal house cat. He was able to communicate with Sirius Black in his Animagus form and was able to understand that Scabbers is in fact a human being in the form of a rat. Crookshanks kept helping Sirius and also helped Hermione and Harry enter the Shrieking Shack by tapping at the right knot at the base of the Whomping Willow.