Have you ever noticed that extra nail on the side of your dog's paw? It might look like a sort of "dog thumb." That's called a dewclaw, and it's a remnant of the evolutionary past of your family pet.
Why Do Dogs Have Dewclaws?

Psychology Today author Dr. Stanley Coren traces your dog's dewclaw back 40 million years to "a tree climbing cat-like animal known as miacis which was an early ancestor of our modern dogs," he says.
"Obviously if you climb trees having five toes is an advantage. However, miacis eventually evolved into the ground dwelling species cynodictus. From this point on, successive generations of the animals that would become our dogs began to become specialized as social hunters," Dr. Coren writes.
That means the extra nail serves little purpose for today's pups. Despite that, most dog breeds still have them on their front paws. Some breeds, like the Great Pyrenees and Briards, have rear dewclaws or can have double dewclaws — called polydactyly.
While they are considered to be of little use, they are certainly not completely useless. Dogs can use them for grip. Most commonly you might see your pup gripping a bone with the help of his "thumb." A dog breed that uses his "thumbs" is a Norwegian lundehund that uses them to help him scale mountains, according to the American Kennel Club (AKC).