I have a 19 year old Appaloosa. I've had him since he was 15 so I don't think this is an age thing. But I've noticed his backbone shows. He's not underweight, he is a thin build naturally but he isn't underweight. He doesn't have any back problems. He's healthy in every way. He had some extra weight on in the winter months and even then I could see his backbone. So I'm guessing it's a breed thing or just the way he's built. I ride him often. Does anyone else have a horse that their backbone shows? It's not super noticeable except certain angles you can notice it more.