Tail Claw

We've recently found a tail claw on our dog. "What is a tail claw?", you ask. Well, it's a claw that grows out of the tip of your animal's tail. I'm wondering how such a thing evolved--the need to scratch his back, or just any place unreachable by the regular claws. It seems unusual enough that my not-so-mini mini schnauzer has a tail (called the Euro tail), but now he has a tail claw. Well, I'll do some research and let you in on what I find. In the meantime here's a photo of tail claw!