A stallion is a male horse that has not been castrated. They are kept for breeding purposes and follow the conformation and phenotype of their breed, but within that standard, the presence of hormones such as testosterone may give them certain physical characteristics. Stallions are also used to refer to males of other equids, including zebras and donkeys.

Stallions can exhibit herd behavior and are sometimes kept in a male-only group of stallions, or, in some cases, with stallions and geldings. Some stallions may develop vicious behavior due to improper socialization or poor handling, and they must be confined and cannot be kept in a natural setting, either because they behave in a dangerous manner toward other horses or because they are dangerous to humans when loose.

Stallions can be managed in multiple systems, depending on the season of the year. 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 also 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 nonhorse companion animal such as a goat, a gelded donkey, a cat, or other creature.

In addition to horses, the term "stallion" can also refer to uncastrated males of other animals, such as dogs or sheep, that are kept primarily as a stud.