– Polydactyl Cats (having more than the normal 5 toes in front / 4 toes in rear) on all four paws is very rare.

– They were at once considered bad luck (instances of witchcraft) and good luck, as sailors would bring them aboard ship for their excellent mousing ability.

–  Teddy Roosevelt had a polydactyl named Slippers.

The Straight Dope (source)