A stallion is a male horse that has not been gelded (castrated). Stallions follow the conformation and phenotype of their breed, but within that standard, the presence of hormones such as testosterone may give stallions a thicker, "cresty" neck, as well as a somewhat more muscular physique as compared to female horses, known as mares, and castrated males, called geldings.
Temperament varies widely based on genetics, and training, but because of their instincts as herd animals, they may be prone to aggressive behavior, particularly toward other stallions, and thus require careful management by knowledgeable handlers. However, with proper training and management, stallions are effective equine athletes at the highest levels of many disciplines, including horse racing, horse shows, and international Olympic competition.
The term "stallion" dates from the era of Henry VII, who passed a number of laws relating to the breeding and export of horses in an attempt to improve the British stock, under which it was forbidden to allow uncastrated male horses to be turned out in fields or on the commons; they had to be "kept within bounds and tied in stalls." (The term "stallion" for an uncastrated male horse dates from this time; stallion = stalled one.) "Stallion" is also used to refer to males of other equids, including zebras and donkeys.
Contrary to popular myths, many stallions do not live with a harem of mares. Nor, in natural settings, do they fight each other to the death in competition for mares. Being social animals, stallions who are not able to find or win a harem of mares usually band together in stallions-only "bachelor" groups which are composed of stallions of all ages. Even with a band of mares, the stallion is not the leader of a herd but defends and protects the herd from predators and other stallions. The leadership role in a herd is held by a mare, known colloquially as the "lead mare" or "boss mare." The mare determines the movement of the herd as it travels to obtain food, water, and shelter. She also determines the route the herd takes when fleeing from danger. When the herd is in motion, the dominant stallion herds the straggling members closer to the group and acts as a "rear guard" between the herd and a potential source of danger. When the herd is at rest, all members share the responsibility of keeping watch for danger. The stallion is usually on the edge of the group, to defend the herd if needed.
There is usually one dominant mature stallion for every mixed-sex herd of horses. The dominant stallion in the herd will tolerate both sexes of horses while young, but once they become sexually mature, often as yearlings or two-year-olds, the stallion will drive both colts and fillies from the herd. Colts may present competition for the stallion, but studies suggest that driving off young horses of both sexes may also be an instinctive behavior that minimizes the risk of inbreeding within the herd, as most young are the offspring of the dominant stallion in the group. In some cases, a single younger mature male may be tolerated on the fringes of the herd. One theory is that this young male is considered a potential successor, as in time the younger stallion will eventually drive out the older herd stallion.
Fillies usually soon join a different band with a dominant stallion different from the one that sired them. Colts or young stallions without mares of their own usually form small, all-male, "bachelor bands" in the wild. Living in a group gives these stallions the social and protective benefits of living in a herd. A bachelor herd may also contain older stallions who have lost their herd in a challenge.
Other stallions may directly challenge a herd stallion, or may simply attempt to "steal" mares and form a new, smaller herd. In either case, if the two stallions meet, there rarely is a true fight; more often there will be bluffing behavior and the weaker horse will back off. Even if a fight for dominance occurs, rarely do opponents hurt each other in the wild because the weaker combatant has a chance to flee. Fights between stallions in captivity may result in serious injuries; fences and other forms of confinement make it more difficult for the losing animal to safely escape. In the wild, feral stallions have been known to steal or mate with domesticated mares.