The horse's stomach is small compared to humans. Their digestive system produces substances to protect the stomach from the effects of the acid and enzymes. For horses, hydrochloric acid is constantly being produced. So, if a horse does not eat, the acid accumulates in the stomach, and can start to irritate the stomach, especially the non-glandular portion. Ulcers are very common for horses, they should be treated often with medicines. We, as their owner should do prevent them to have ulcers because it's a very fatal thing for them. I remember when my horse (Garret) had an ulcer and I didn't have any idea what to do that time. Good thing the internet that time was alive and found myself solutions (probably bought some medicines over) on how to prevent it. God, the internet sure is useful. Haha.