Polydactyl

Most cats have 18 toes; five on their front paws and four on their back ones; but some cats have an extra digit or two—they’re known as polydactyl cats. Experts estimate as many as 40% of early Maine Coons were polydactyls, which some people said helped the cats use their paws as “natural snowshoes” during snowy Maine winters. Although the mutation can spontaneously arise within any cat population, the trait is most common in the eastern U.S. and Canada, as well as England. This is believed to be attributed to the popularity of polydactyl cats on ships; sailors thought they were good luck and that their extra toes provided them better balance on the high seas—and allegedly gave them superior mouser capabilities.  

This trait is a natural genetic variation and is considered a purebred characteristic, not a health issue.