Vomit is a defensive reaction from the body that allows any harmful intake to be removed from the stomach, however the horse lacks this option. Horses can't vomit because they possess a valve at the entrance of the stomach called cardias or " Swiss tie ", the muscles of this valve are so strong that they prevent food from returning to the mouth. In humans and most animals, in case of poisoning or strong indigestion, your body produces the vomit to release the stomach, however this does not happen on horses, as the above valve causes the ingested to pass directly from the stomach to the intestines, polluting them and provoking the horse terrible colic which in most cases ends up in the animal's death. What's stopping horses from throwing up? To the question why don't horses vomit? the answer is that: the anatomy of the equine is the one that makes vomit impossible. Their digestive system is designed in such a way that food can enter but not exit, just like they can't breathe through their mouth. Knowing this, there's another recurring question that is usually: horses don't breathe while eating? The answer is no. Only while they swallow the food though.