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.
A stallion
Mustang stallion (right) with part of his band of mares and foals
Stallion exhibiting the flehmen response
A stallion's secondary characteristics include heavier muscling for a given breed than is seen in mares or geldings, often with considerable development along the crest of the neck, as shown in this image.
A gelding is a castrated male horse or other equine, such as a pony, donkey or a mule. The term is also used with certain other animals and livestock, such as domesticated camels. By comparison, the equivalent term for a castrated male bull would be ox, or a wether for rams and billy goats.
A 3-year-old gelding
Gelding a male horse can reduce potential conflicts within domestic horse herds.
An open castration being performed on a horse under ketamine anaesthesia
Recumbent castration, including use of emasculators
