Dewclaw

A dewclaw (commonly referred to as a Dog's thumb) is a vestigial digit on the foot of many mammals, birds, and reptiles (including some extinct orders, like certain theropods). It commonly grows high on the leg so that in digitigrade species, when the animal is standing, it does not make contact with the ground. Dewclaws are perhaps best known in dogs, and they are often removed in young puppies, though there is debate on whether this is necessary.