Hay belly describes the physical appearance of a distended abdomen on a horse. The belly area looks puffy and round, sticking out to the sides and hanging low. If your horse has a big belly and little muscle, it is likely the hay you provide is of poor quality and is contributing to his belly condition. There are many high-quality diets available that your veterinarian will be able to recommend. For example, if your horse is on pasture, your vet might recommend supplementing its grazing with high-quality hay and a protein supplement in the form of a concentrated feed. If you are feeding your horse a combination of grain and hay, your vet can suggest a diet containing a certain ratio of each to improve the condition.