dewclaw
A dewclaw (commonly referred to as a dog's thumb) is a vestigial anatomy on the foot of many mammals, birds, and reptiles. It commonly grows high on the leg such that in digitigrade species, when the animal is standing, it does not make contact with the ground. You've probably seen one on your dog or cat. If the claw is lower, on the back of the heel, it's likely a heel claw instead.
