Medieval warhorses were bred and trained to hold formation despite battle chaos

Evidence

A knight’s warhorse was the most important, and by far the most expensive, tool of the lot. The best warhorses were bred for the task, tall, strong, fast, and reliable. The rarest and most highly prized were called “destriers.” These animals were trained from an early age to respond instantly to the slightest commands, and to walk in a perfectly straight line. They had to hold course despite extraordinary distractions: screams and howls of battle and foot soldiers rushing them swinging axes and clubs. A horse that balked at the moment of battle was a dangerous liability. Breeding and training the best horses cost a fortune, and the wealthiest knights had three or four in their retinue. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 348