I met a cat once that appeared to have thumbs. Turns out it isn't that rare for cats to be born with extra toes. It may be considered a freak of nature but I think just one extra toe is neat looking and the cats can use this sort of toe/thumb to do things they maybe could not have done before. Or maybe it's a form of cat evolution.
http://www.thecatgallery.com/polydactyl_cats.html
http://www.thecatgallery.com/polydactyl_cats.html