It is a known fact that horses cannot vomit. Why?
They have a band of muscle around the esophagus as it enters the stomach. This band operates in horses just like in humans: as a one-way valve. Food freely passes down its esophagus into the stomach as the valve relaxes but the valve squeezes down the opening and cuts off the passage for food going back up.
Their valve however, unlike humans, really works. Horses physically can’t because of the power of the cut-off valve muscle. Also, the esophagus meets its stomach at an angle which enhances the cut-off function when its stomach is bloated with food or gas.