My daughter brought home a stray cat a few months ago and it has "Thumbs." It is actually 2 dew claws, one is 'extra' and large on each paw. The cat seems to be able to use these extra toes, not as well as an actual thume.
His name is Gizmo, but I call him 'eeeow'... I asked him his name and he said, "eeeow" so I took him at his word.
Take a look..... This is the only one I have seen (but I am hearing they are common)