Normally, dew claws are only found on front legs - they're normally "docked" at the same time the tail is. There are some who will decry the practice as barbaric, but the presence of dew claws can cause a lot of pain and discomfort when they get snagged - and they WILL.
Sometimes a dog will have dew claws on the hind legs, too. It doesn't happen often, but it can occur.
I know some cats have extra toes: their feet look like they have boxing gloves on
. There is a whole colony of them in Key West; they're decendents of Ernest Hemingway's cat. The technical name is "polydactyl" ... a cuter name is "mitten foot".