Work, work and more work. And if you want correct muscles, you need correct work, using the horse's hind legs and topline.
Running around with their head in the air and hind legs not going anywhere will build up shoulders and under muscle of the neck.
You CAN supplement for muscle building when the horse is in heavy work - I give my competition horse Kohnke's Muscle XL in a slurry form after a strenuous session to aid recovery.
But a supplement alone will never build muscle tone.
Just like in humans, you can eat all the protein you want, but if you're sitting on the coach all day it's not going to build muscle.