Some polydactyl kittens and cats that have six toes on their front paws have the extra digits on opposing sides of the paws, rather like human thumbs. (best-cat-art.com)
In a commercial for the Xperia Play, the PSP Phone, the adorable robot gets human thumbs sewn onto him in a seedy back alley. (gizmodo.com)
Dewclaws are found above the paw's toes and are similar to human thumbs except they aren't functional. (animalbehaviorcollege.com)