manx (cat)

The Manx is a breed of cats with a naturally occurring mutation of the spine. This mutation shortens the tail, resulting in a range of tail lengths from normal to tailless. Many Manx have a small 'stub' of a tail, but Manx cats are best known as being entirely tailless and it is the distinguishing characteristic of the breed and a  cat body type genetic mutation.
See more at Wikipedia.org...