Stallions will 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.
When the herd is in motion, the stallion brings up the rear, keeping straggling herd members closer to the group and serving 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 usually is on the edge of the group, to defend the herd if needed.
In a herd or a group of horses, there is usually one mature herd stallion. Both sexes of horses are tolerated while young, but once they become sexually mature, 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. 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, but colts or young stallions without mares of their own usually form "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.
The internal genitalia comprise the accessory sex glands:
Domesticated stallions are trained and managed in a variety of ways, depending on the region of the world, the philosophy of the owner, and the temperament of the individual stallion. In all cases, however, stallions have an inborn tendency to attempt to dominate both other horses and human handlers, and will be affected to some degree by proximity to other horses, especially mares in heat. They must be asked to behave with respect toward humans at all times or else their natural aggressiveness, particularly a tendency to bite, may pose a danger of serious injury.
For this reason, regardless of management style, stallions must be treated as individuals and should only be handled by people who are experienced with horses and thus recognize and correct inappropriate behavior before it becomes a danger. While some breeds are of a more gentle temperament than others, and individual stallions may be well-behaved enough to even be handled by inexperienced people for short periods of time, common sense must always be used. Even the most gentle stallion has natural instincts that may overcome human training. As a general rule, children should not handle stallions, particularly in a breeding environment.
Management of breeding stallions usually breaks down into one of two basic types: confinement or "isolation" management, and natural or "harem" management. Sometimes a stallion may periodically be managed in both systems, depending on season of the year. In some parts of the world, young domesticated stallions may be allowed to live separately in a "bachelor herd," out of sight, sound or smell of mares.
Natural management is essentially allowing a stallion to run in a pasture with a herd of mares. The advantage of this type of management is that the stallion is allowed to behave "like a horse" and may exhibit fewer stable vices. In addition, the mares may "cycle" or achieve estrus more readily. Proponents of natural management also assert that mares are more likely to "settle" (become pregnant) in a natural herd setting. Some stallion managers keep a stallion with a herd year-round, others will only turn a stallion out with mares during the breeding season.
There are drawbacks to natural management, however. One is that the breeding date, and hence foaling date, of a given mare will be uncertain. Another problem is the risk of injury to the stallion or mare in the process of natural breeding. Some stallions become very anxious or temperamental in a herd setting and may lose considerable weight, sometimes to the point of a health risk. Some may become more aggressive and dangerous to handle. There is also a greater risk that the stallion may escape from a pasture or be stolen. Stallions may break down fences between adjoining fields in order to fight another stallion, or possibly mate with the "wrong" herd of mares, thus putting the pedigree of ensuing foals in question.
The other general method of managing stallions is to confine them individually, separately from other horses, sometimes in a small pen or corral with a tall fence, other times in a stable, or, in certain places, in a small field (or paddock) with a strong fence. The advantages to confinement include less of a risk of injury to the stallion or to other horses, controlled periods for breeding mares, greater certainty of what mares are bred when, less risk of escape or theft, and ease of access by humans. Some stallions are of such a temperament (or develop vicious behavior due to improper handling) that they must be confined and cannot be kept in a natural setting, either because they behave in a dangerous manner toward mares, or because they are dangerous to humans when loose.
The drawbacks to confinement vary by the actual method used, but lack of exercise can be a serious concern; stallions without sufficient exercise may not only become fat, which may reduce both health and fertility, but also may become aggressive or develop stable vices due to pent-up energy. If stallions are kept in complete isolation from other animals, they may develop additional behavior problems. Conversely, some stallions within sight or sound of other horses may become aggressive or noisy, calling or challenging other horses. In any case, stallions kept alone require careful balance of nutrition and exercise for optimal health and fertility.
As a general rule, a stallion that has been isolated from the time of weaning or sexual maturity will have a more difficult time adapting to a herd environment than one allowed to live close to other animals.
Some managers attempt to compromise between the two methods by providing stallions daily turnout by themselves in a field where they can see, smell, and hear other horses. They may be stabled in a barn where there are bars or a grille between stalls where they can look out and see other animals. In some cases, a stallion may be kept with or next to a gelding or a non-horse companion animal such as a goat, gelded donkey, cat, or other creature.
Properly trained stallions can live and work close to mares and to one another. Examples include the Lipizzan stallions of the Spanish Riding School in Vienna, Austria, where the entire group of stallions live part-time in a bachelor herd as young colts, then are stabled, train, perform, and travel worldwide as adults with few if any management problems. However, even stallions who are unfamiliar with each other can work safely in reasonable proximity if properly trained; the vast majority of Thoroughbred horses on the racetrack are stallions, as are many equine athletes in other forms of competition. Stallions are often shown together in the same ring at horse shows, particularly in halter classes where their conformation is evaluated. In horse show performance competition, stallions and mares often compete in the same arena with one another, particularly in Western and English "pleasure"-type classes where horses are worked as a group. Overall, stallions can be trained to keep focused on work and may be brilliant performers if properly handled.
A breeding stallion is more apt to present challenging behavior to a human handler than one who has not bred mares, and stallions may be more difficult to handle in spring and summer, during the breeding season, than during the fall and winter. However, some stallions are used for both equestrian uses and for breeding at the same general time of year. Though compromises may need to be made in expectations for both athletic performance and fertility rate, well-trained stallions with good temperaments can be taught that breeding behavior is only allowed in a certain area, or with certain cues, equipment, or with a particular handler. However, some stallions lack the temperament to focus on work if also breeding mares in the same general time period, and therefore are taken out of competition either temporarily or permanently to be used for breeding. When permitted by a breed registry, use of artificial insemination is another technique that may reduce behavior problems in stallions.
People sometimes have inaccurate beliefs about stallions, both positive and negative. Some beliefs are that stallions are always mean and vicious or uncontrollable, other beliefs are that misbehaving stallions should be allowed to misbehave because they are being "natural," "spirited," or "noble." In some cases, fed by movies and fictional depictions of horses in literature, some people believe that a stallion can bond to a single human individual to the exclusion of all others. However, like many other misconceptions, there is only partial truth to these beliefs. Some, though not all stallions can be vicious or hard to handle, occasionally due to genetics, but usually due to improper training. Others are very well-trained and have excellent manners. Misbehaving stallions may look pretty or be exhibiting instinctive behavior, but it can still become dangerous if not corrected. Some stallions do behave better for some people than others, but that can be true of some mares and geldings as well.
In some parts of Asia and the Middle East, the riding of stallions is widespread, especially among male riders. The gelding of stallions is unusual, viewed culturally as either unnecessary or unnatural. In areas where gelding is not widely practiced, stallions are still not needed in numbers as great as mares, and so many will be culled, either sold for horsemeat or simply sold to traders who will take them outside of the area. Of those that remain, many will not be used for breeding purposes.
In Europe, Australia, and the Americas, keeping stallions is less common, primarily confined to purebred animals who are usually trained and placed into competition to test their quality as future breeding stock. The majority of stallions are gelded at an early age and then trained for use as everyday working or riding animals.
Just as some pet owners may have conflicting emotions about neutering a male dog or cat, some stallion owners may be unsure about gelding a stallion. However, in most cases, particularly in modern industrialized cultures, an animal that is not of sufficient quality to be used for breeding will have a happier life without having to be isolated or constantly dealing with the instinctive, hormone-driven behaviors that come with being left intact. Geldings are safer to handle and present fewer management problems. They are also more widely accepted. Many boarding stables will refuse clients with stallions or charge considerably more money to keep them. Some types of equestrian activity, such as events involving children, or clubs that sponsor purely recreational events such as trail riding, may not permit stallions to participate.