How much feed are you feeding and how big and what age are your horses?
You're not going to find many feeds higher in fat than Ultium...
Add a 11.7% protein is not terrible either and adequate amount to build muscle for most horses.
Protein...well, it isn't only protein that makes muscle tissue..you need the corresponding ingredients, building blocks in proper amounts.
Do your research of what building blocks are needed and in the right ratios. Figure out what you might be missing and need and then add that to the diet.
I have had great success adding the bloom to my rescue with a product from Pennwoods called Body Builder 4000.
My horse is not tattooed but sure looks a older Thoroughbred.
Has made a marked difference in his appearance.
At 26% protein and 40% fat I don't think there is anything like it on the market.
The supporting ingredients are also there. Concentrated small amount needing fed daily... https://pennwoods.com/body-builder-4000/