The Kneazle is a very intelligent cat-like creature which can detect unsavory or suspicious persons very well and will react badly to them. However, if a kneazle takes a liking to a witch or wizard, it makes an excellent pet. The Kneazle has spotted fur, large ears, and a lion-like tail (FB). Crookshanks is part Kneazle (Nr, JKR).
Kneazles seem to have a sixth sense about good and evil. For instance, Crookshanks saw through Peter Pettigrew's disguise as Scabbers and attacked him from the first time he saw him. Sirius as an Animagus dog spoke to the cat/kneazle outside the Shrieking Shack and then Crookshanks threw himself on Sirius's chest to protect him from Harry's wrath as though he knew Sirius was innocent (CS17).
Kneazle is pronounced NEE-zul.
Although Mrs. Norris has a Kneazle-like personality, J.K. Rowling has confirmed via twitter that Mrs Norris is not part Kneazle.