Can you really increase your horse's muscle mass by feeding it the right nutrients? Yes, to an extent. The one key requirement more than what you feed is that a horse won’t develop more muscle unless it is in regular training. Even anabolic steroids work best when used in people or animals that are exercising. Of course, anabolic steroids are banned from being used in horses except in rare cases where they are used for specific medical reasons. Interestingly, a study in horses showed that the use of anabolic steroids in yearlings did increase muscle mass but by . . .