The Alpha Horse is the dominant leader in a herd. Horses instinctively seek leadership. The Alpha Horse provides leadership and security in a herd. It is quite common for the Alpha Horse to be an old mare." [source]
When anxiety over separation occurs while a horse is being handled by a human, the horse is described as "herd-bound". However, through proper training, horses learn to be comfortable away from other horses, often because they learn to trust a human handler, essentially ranking humans as a dominant member of a herd." [source]