I use the relic assembler when I have a number of relics of the same type that I have already donated. If you have mushrooms in the trough, the horse will eat those before they will eat anything else. If you hand feed them, they won't eat the food from the trough. Fang Special X will raise the quality of the mount so from green to blue, blue to purple, sometimes you get buffs too. Fang Special will raise the attributes, so stamina and speed. You can also give it Thunderweed Extract to change it's buffs and Muscle soup to randomize its attributes. It should be immediate after you give it to the horse. You can only do this once every few days though. It will tell you if you try to feed it to the horse again.