Q: It is a known fact that horses cannot throw up. Why? A: The vets agree with you: horses cannot throw up. That's what they are taught in veterinary school; that's what the books say. Horses have a band of muscle around the esophagus as it enters the stomach. This band operates in horses much as in humans: as a one-way valve. 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.
What can I say ? We don't care :)