I knew that this would apply to either western or english. I have a 5 year old gelding that I ride 6 days out of the week. He is in perfect shape, very* healthy. I don't have any inclines, hills or sand. I want to know how to fill in that place on his rear end with muscle. I ride western if that helps. P.s. I do back up everyday numerous times because it was suggested to me by an old cowboy friend of mine.
If he's broke to rope, you can drag logs or anything else heavy off him at a walk. That builds their butt up just as well as working hills.
Though if he's not broke to rope, I wouldn't suggest trying it without training him to rope. I've been hurt bad a couple times trying to work a horse on the rope that either wasn't ready or wasn't broke (like I was told he was LOL).
For example, this tire is what I use to "log" my horses in preparation of roping.