Why do horses live in groups?

Wild horses were not always in the American West. The ancient ancestors of horses that were developing in North America died out during the ice age. A large portion of the population of these pre-horses (Eohippus, Mesohippus, and others, depending on the stage of evolution) migrated over a land bridge through Alaska and into Asia, where they finished their development and evolved into the horses we know today. They were domesticated throughout Asia and Europe for many years before the Spanish explorers accidentally introduced them into North America. Horses were brought over by ships to use for exploration of the New World, and many escaped. The vast expanse of grassland in the American West proved to be the perfect environment for the horse population to grow, and after generations adapt to become the American Mustang.