The horse (Equus caballus) is one of ten species in the genus Equus (which includes zebras, donkeys, onagers, etc...). Horses have played an important role in history, they are used for transport, working the fields, warfare, etc... It is estimated that the domestication of horses began around 4500 BC, but there are competing theories. The Przewalski horse is the only living wild horse species on the planet today. Wild horses are different from feral horses in that they have never undergone domestication. Did you know mules are cross-breeds between a male ass and a mare and it is infertile? Recently breeders have cross-bred zebras with mares or female asses to produce "zebra mules" or "zorses", though the animals keep some of the nervous, difficult to handle nature of zebras. The difference between ponies and horses is only their size. The treshold is 1.47 m for an adult. It can happen that a pony is actually the child of a horse and vice versa.