You can buy oats and mix with molasses and any fruits/veggies your horse likes. Sometimes I add applesauce to it too, then make cookie shapes on a cookie pan and bake them until they turn golden.
You should take a look at this article, there are 7 recipes :) http://saratogastalls.com/7-horse-treats-to-pamper-your-horse.html
I like simple things so the fruit salad is my favorite "recipe". Just mixing some fruits or vegetables and it's done.
You can do a smoothie (I know it seems strange). It's very easy: cut the fruits or vegetables of your choice and mix them in a blender. It's ready! You can add some water if you want the smoothie to be more hydrating!
Sometimes I cut apples and carrots and add some honey on top of it. My horse seems to think it's delicious!
You can cut many fruits such as apples, bananas, strawberries, pears and then mix everything: your fruit salad is ready and it's the easiest treat to do ever!