Try pole work! I have a 4 year old thoroughbred as well. It helps not only to form the muscles but also it helps to connect with the horse and practice f.eg. Turns, control, balance etc.
This isn’t a riding exercise, but There’s this thing called the “Pessoa lunging system.” I always lunge my horse in it and it works so perfectly!! She’s gained lots of muscle and strength, but has also learned to stay soft while riding!