Found this informative article on Pilchuck Veterinary Hospital web site: 
Research studies estimate that 60% to 90% of performance horses have gastric (stomach) ulcers. What signs might they exhibit? Horses with gastric ulcers can present in a variety of ways: weight loss/hard keeper, poor hair coat, bad behavior, poor performance, low-grade colic, diarrhea, poor appetite.
The equine stomach has two distinct regions: the non-glandular (squamous) region and glandular (acid-producing) region. Ulcers can form in either region, but most occur in the non-glandular tissue.
Stomach ulcers are the result of multiple factors, including infrequent feeding, prolonged travel and intense exercise. Most importantly, horses continuously secrete stomach acid 24 hours per day. From an evolutionary standpoint, this makes sense. Horses are designed to graze continuously, which leads to direct buffering of the acid by feed material and bicarbonate-rich saliva. However, modern-day equine management often results in horses only eating two to three meals per day, putting our equine companions at significant risk for the development of gastric ulcers. Research has also shown that the size of the equine stomach decreases during exercise, which results in the delicate non-glandular portion of the stomach being in contact with acid for a prolonged period of time. Finally, the use of non-steroidal anti-inflammatory agents (such as “Bute” or Banamine) can cause gastric ulcers.
Currently, the only reliable method for diagnosing equine gastric ulcers is with gastric endoscopy. This procedure allows the veterinarian to directly visualize the stomach for the presence of ulcers or other abnormalities. Once diagnosed, gastric ulcers are very effectively treated with GastroGard (omeprazole).
– by Wendy Mollat, DVM, DACVIM, who is specialized in equine gastrointestinal issues