What do horses eat beside eating hay?

You are better feeding hay plus a pellet, supplement if needed, and a good vitamin and mineral. You want a grass or rice based pellet. The processed grains such as Purina products, and Ranchway products have nothing but sugar in them and it'll make your horse go nuts. Fat based such as Envision are always better than protein products such as alfalfa. Fat has 2.5x the energy that protein does and it's a lot easier on your horse's body. If you ever walk into a barn and smell ammonia, that's extra protein that's leaving the body through urine, VERY harmful on the kidneys. Grass and rice based pellets are best.

Well hay and grass are the basis of the horses diet and should never make up less than 90% of their total daily food unless their health requires it, such as bad teeth, ect(pellets do not count as "grain." The grain we are talking about is processed grains). It is best however to ask an equine nutritionist what other feeds would be best for your individual horse as not all horses can or should be fed the exact same things.