Oats have been used for 2000 years as a raw material for horse feed and give sport horses extra quickly available energy. Oats, unlike many other grains, contain very fast and easily digestible protein which in many cases is already digested in the stomach and thus does not enter the large intestine.
Oats also contain Tryptophan which releases endorphins and makes the horse happy. In some cases a little too much making the horses harder to ride.
Flattened oats digest quickly because here the husk around the oats is broken, it is a little less potent than unflattened oats though.
Black and white oats are virtually the same in nutritional value and are both very suitable as horse feed. However, the quality of white oats can be slightly more variable and usually white oats are cheaper.