A high fat and high amino acid feed. You may also have to mix two feeds to get the weight you want. That's what I had to do with my OTTB when he wasn't putting on weight. I mix a high fat feed with a high fiber feed.
The equine omega complete is amazing I started using it after I saw Claire Eventing use it you can check her out in instagram and YouTube she has an OTTB horse as well and he was in a poor condition but now he looks sooooooo good you can ask her for all the details I'm not that good in explaining 😁😁😁