I made some for our dogs using pumpkin puree and cooked sweet potato along with peanut butter. I think horses would probably like these, as well. You could probably also add some cooked carrots or apples and swap out the peanut butter for molasses. But, I think horses generally like peanut butter so they will probably eat them as is.
4 cups spent grain
3 cups flour
1 cup peanut butter
3/4 cup pumpkin (more or less depending on consistency)
1/4 cup sweet potato
Mix all ingredients in large bowl until about the consistency of thick cookie dough. Spoon out in individual cookies based on your dog size onto cookie sheets... For horses, I'd say just make large bars.
Cook in 325 oven until brown (probably 30 - 45 mins), keep warm in low oven (170) until solid (several hours).
Hope that helps.