A horse is a type of mammal that has long had a close association with humanity. Some experts believe that the first domestic horses were companions to Asian nomads several thousand years ago. Today, there are several breeds of domestic horses, but all belong to one species, Equus caballus.
In addition to domestic horses, there are also wild horses and feral horses. True wild horses do not have domestic ancestors, whereas feral horses are wild horses that are descendants of domestic horses. As of 2015, the only true wild horse, which is kept alive in captivity, is the Przewalski's horse. This horse does not claim domestic ancestors, but is also believed to be extinct in the wild. An example of a feral horse is the North American mustang, which comes from domesticated ancestors that traveled to the Americas during the time of European exploration.
Wild horses often form groups of around three to 20 individuals, including a male leader called a stallion, as well as a number of female mares and young foals. Domestic horses, which number several hundred breeds, have performed many services for humanity throughout history, including hauling wagons and other heavy equipment, racing and carrying soldiers into battle.