What's everyone's go-to for building muscle? I prefer something halfway affordable lol ( DONT WE ALL) 

These are performance horses who get worked regularly doing  Long trotting, backing, moving over poles, and working small hills. 

Just wondering what I could add to help aid in keeping that muscle or getting bigger ?

JMHO but I feel muscle tone is built by working the horse and a good feeding program instead of a supplement. Also I feel some horse's are born with more muscle than others. Abbie was born with muscle more so that my mare Heidi. The only supplement abbie is on is Suceed and she gets her twice a year adaquan dose. God just made abbie with alot of bone.

Quality nutrition thru forage and feed with proper amino acids for building muscle, quality exercise, and one of my favorites THE Muscle Mass supplement. 

• Edited to add.. also staying on top of Dental work goes a very long way! We can feed the highest quality hay and feed and it won't do a bit of good if they can't properly chew it. (Currently dealing with this) 

Use Muscle Mass. 

With excercises and nutrition.