I had the same problem with an appendix quarter horse that I got out of a neglect situation. He always has energy, but I don’t think any particular food or supplement is going to “fix” that; I just know that he will require a couple of good lunges before our class. He needed to gain about 400 lbs when we got him (lack of food and lots of worms), and the best combo for him was Buckeye EQ8 and Ultimate Finish 40. Both of these are high fat and low starch so it won’t add to his energy level. Just as a heads up, it did take about 1.5 years to get all that weight on him, so don’t expect overnight results.