Horse feed is an important part of a horse's diet and can include OATS, HAY, and MASH. OATS are a type of cereal grain that is high in fiber and protein, and is often used as a horse feed. HAY is a type of grass or legume that is cut and dried for use as animal feed. MASH is a type of feed made from grains, such as OATS, that have been ground and mixed with water or other liquids. All three of these items are commonly used to feed horses.