Feeding too much can lead to several health problems, including obesity, laminitis, and colic. Obesity can put stress on a horse's joints and cardiovascular system and make them more susceptible to disease. Laminitis is a painful condition that can cause the coffin bone to sink and rotate within the hoof. Colic is a general term for abdominal pain that can be caused by a variety of factors, including too much food or an abrupt change in diet.
If you are concerned about your horse's weight or diet, consult with a veterinarian or equine nutritionist.