A stallion is an intact adult male horse — one that has not been castrated — typically considered to have reached full sexual maturity at four years of age or older. Stallions retain the hormonal profile driven by endogenous testosterone, which produces a characteristically heavier muscular build, a thicker crest on the neck, and a more reactive temperament compared with the gelding or mare.
The primary purpose for keeping a stallion intact is breeding. A proven stallion with desirable conformation, athletic record, or bloodline commands significant stud fees and may cover dozens of mares in a single breeding season through live cover or collected and shipped semen. Breed registries maintain detailed records of stallion registration, progeny performance, and inspection criteria for breeding approval.
Managing a stallion requires facilities designed for containment and separation from other horses, particularly mares in estrus. Turnout with other horses is generally limited or carefully supervised, since inter-horse aggression is markedly higher among intact males. Many facilities house stallions in stalls with solid walls or double-fence paddocks to prevent contact-related injury.
A male horse younger than four years is a colt rather than a stallion, though the threshold varies by breed registry. Once castrated at any age, the horse becomes a gelding regardless of prior breeding history.
Further Reading
For formal stallion registration and pedigree documentation in Thoroughbred racing: