My mom's horse is a 15 hand tall quarter horse. He's about ten years old and has always been well-built. The previous owner moved to another state and left him and another of our horses back here and my mom fed them every day for about two years before they became ours. That whole time she was feeding them they usually went without hay. Last year they gave both horses to us and they got hay and grain every day, including the grass in the pasture in the spring, summer, and fall. In the past month he's dropped at least fifty pounds, maybe one hundred. He's gone from muscular and built to ribby and sunk in. What could be wrong with him? He gets wormed every month.