The dewclaw – those short claws or nails on the side of the dog’s foot which doesn’t touch the ground.  They are usually found only on the front paws, rarely on the back paws.  However, rear dewclaws are common in such breeds as the Great Pyrenees and Briards with the Great Pyrenees having a double dewclaw.  This is an inherited trait called “polydactyl.”
Dewclaws are nonfunctional in most dogs but they are an interesting bit of their evolutionary past.  About 40 million years ago, a tree climbing cat-like animal called “miacis” was an early ancestor of our modern dog.  Eventually, miacis evolved into a ground dwelling species known as “cynodictus.”  Successive generations began to become social hunters of fast moving prey making speed an important factor.  Dogs of today are a “cursorial” species meaning, through evolution, they are swift runners but this added speed needed a change in the canine physiology.
Humans and bears are “plantigrade” animals which means the full length of the foot is placed on the ground then move with a rolling heel to toe action.  Though balance and stability is good, the process was slow.  So evolution rocked the dog’s legs forward so that their heels no longer touched the ground making them a digitigrade species, which means they walk on their digits.  Along with longer and stronger forelegs, dogs had the speed they needed for fast prey.  Due to these physical changes, the sole of the dog’s foot never touches the ground and the dewclaw is too short to be of any value.
Dog owners are cautious of dewclaws, both front and back, as they can catch on something during a run over rough terrain which can tear the dewclaw off and cause serious injury.  For that reason, some breeders remove the dewclaw from puppies before they are adopted out however, most dogs have their dewclaws intact.
There is an interesting bit of southern folklore that keeps some people from removing the dewclaw.  It is believe that if a dog has dewclaws on their hind feet, which is rare, they have a natural immunity to the poison of a venomous snake bite as long as the dewclaw stays intact.  They believe “them dewclaws suck up the poison.”