Fly Cyst ? I have NO idea, never seen anything like it before. It's on my horse's back, right along his spine. It's been there for at least a month, and it looks like it's getting smaller. He's really underweight and he's gaining it back fast. The more weight he's gaining the more it's going away. When I first got him you could see the his back bone, that one was just 3 x the size of the other bone "bumps". There's also grey hairs growing out of them now that weren't there before. Suggestions?