My appaloosa has a similar problem–with the dark colored ‘skin’ rubbing off, very thin skin/hair coat, and easily developing sores. He also gets rubbed easily under his saddle pad. I have only had one vet look at this, and he gave me a medicated shampoo. This helped, but had to be done everyday. As soon as I stopped using the shampoo, the skin issue returned. I’m going to have someone else look at him to get a second opinion, but please, if you have any suggestions or find anything out, share it! Hope you figure out what the problem is!