Horses don’t like living alone. Many will tolerate it, but all prefer to be with other horses. Like humans, horses are very social animals. In fact, in a way, horses are more social than humans, because for them, the company of their own kind is perceived as an issue of life or death. In the wild, a horse living alone is more vulnerable to predators. There is safety in numbers, and horses know it.
Horses also get very lonely without other horses around. Some experts have observed that among wild horses, young stallions driven from their family herds by older, more dominant stallions were visibly depressed and forlorn. These solo young stallions wander aimlessly, as if life isn’t worth living. Rather than go on alone, some of them join up with other outcasts to form “bachelor bands.” While the goal of every stallion is to have a herd of mares to call his own, stallions would rather live with other stallions . . . their potential rivals . . . than be by themselves.
Page 71 “Why do horses sleep standing up”