A mare and her foal.
Horses have a band of muscle around the esophagus as it enters the stomach.
Food freely passes down the 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. Horses, however, differ from us because their valve really works. Humans can vomit. Horses almost physically can't because of the power of the cut-off valve muscle. Also, the esophagus meets the stomach at an angle which enhances the cut-off function when the horse's stomach is bloated with food or gas.