I do this for my dogs:
Mix equal parts moist grain and flour to create a dough like consistency.
Add flavoring if necessary (I use peanut butter for dogs, not sure for horses)
Add an egg or two based on batch size (I find they stick together enough regardless)
Roll out onto a baking sheet (<=0.5" thick). Bake at 350F for an hour, reduce to 200F for a few hours, when baked hard they are done, watch for burning.
Great for dog treats, as you can cut them up based on the size of your dogs, but if I had horses, I'd just give them shovels full of grain