The Horse, Equus caballus, is a large ungulate mammal, one of the seven modern species of the genus Equus. It has been important for transportation; to ride on, or for pulling a chariot, carriage, horse-drawn boat, stagecoach, tram, etc.; also as plough horse, etc. as well as food; see also domestication of the horse. Until the mid 20th century, the horse was used heavily in warfare, where the machines who now take the place of the horse on the battlefield are still called cavalry units, sometimes keeping traditional names (Lord Strathcona's Horse, etc.)
Domestication of the horse and surviving wild species
The earliest evidence for the domestication of the horse comes from Central Asia and dates to about 3,000 BCE. Competing theories exist about the time and place of domestication. However, wild species continued into historic times, including the Forest Horse, Equus caballus silvaticus (also called the Diluvial Horse); it is thought to have evolved into Equus caballus germanicus, and may have contributed to the development of the heavy horses of northern Europe, such as the Ardennais.
The Tarpan, Equus caballus gmelini, became extinct in 1880. Its genetic line is lost, but a substitute has been recreated by "breeding back", crossing living domesticated horses that had features selected as primitive, thanks to the efforts of the brothers Lutz Heck (director of the Berlin zoo) and Heinz Heck (director Tierpark Munich Hellabrunn). The resulting animal is more properly called the Wild Polish Horse or Konik.
The only true surviving wild-horse species is Przewalski's Horse, Equus caballus przewalskii przewalskii Polaikov, a rare Asian species. In Mongolia it is known as the taki, while the Kirghiz people call it a kirtag. There are wild populations in Mongolia, see: http://www.treemail.nl/takh/.
Wild vs. feral horses
One can distinguish between wild animals, whose ancestors have never undergone domestication, and feral animals, whose ancestors have been domesticated, but who now live in the wild. There are several populations of feral horses, including those in the West of the United States (often called mustangs) and in parts of Australia (called brumbies) and in New Zealand called Kaimanawa horses. These feral horses may provide useful insights into the behavior of their ancestral wild horses.
The Icelandic horse (which has the size of a pony but is referred to as a horse) is an interesting breed from a historic and behavioural point of view. Introduced by the Vikings into Iceland, they have not been subject to the selective breeding that has taken place in Europe from the middle ages until now, giving us a picture of what horses looked like and behaved like in those times. The Icelandic horse has a four-beat gait called the Tolt, which equates to the Rack exhibited by several American gaited breeds.
Other members of the horse family include zebras, donkeys, and hemionids. The Donkey, Burro or Domestic Ass, Equus asinus, like the horse, has many breeds. A mule is a hybrid of a male ass and a mare and is infertile. A hinny is the less common hybrid of a female ass and a stallion. Recently breeders have begun crossing various species of zebra with mares or female asses to produce "zebra mules"—zorses and zedonks. This is likely to remain a novelty hybrid as these individuals tend to inherit some of the nervous, difficult nature of their zebra parent.