The three species of zebra belong to the family Equidae. Zebras are equines and closely related to horses and donkeys. This family has several surviving species besides zebras, including wild horses, feral donkeys and wild asses. Zebras are more distantly related to other members of their order Perrisodactyla, a group of herbivores that includes rhinoceroses and tapirs.
Przewalski's wild horse (Equus ferus przewalkskii) belongs to the same species as the familiar domestic horse, although it is a genetically separate subspecies. The species was extinct in the wild until reintroduction efforts began in the 1990s. Wild herds are now in Mongolia, and attempts to get wild populations established in China, Khazakstan and the Ukraine are ongoing. The International Union for the Conservation of Nature still lists Przewalkski's wild horses as "critically endangered," and there are only about 50 truly wild individuals as of 2011.
Humans domesticated the horse (Equus ferus caballus) over 5,000 years ago, primarily as a working animal, although its meat is edible and consumed in some countries to this day. Several feral populations of once domestic horses have reverted to the wild. Examples include the mustangs of North America and the brumbies of Australia.
The donkey (Equus africanus) has a few surviving wild populations in Asia and Africa and is a widely bred domestic animal, with several feral populations. The African wild ass is probably the ancestor of the domestic donkey. While domestic donkeys are widespread throughout the world, the wild forms are endangered.
The kulan, or Asian wild ass (Equus hemionus) is native to southeast Asia, in particular Mongolia, although its range was far wider in the past, extending into Europe. Kulans are endangered because of habitat destruction, competition with livestock for water and food, and hunting for meat. Their population is still declining.
The kiang or Tibetan wild ass (Equus kiang) lives in mountainous habitats of Tibet and its range extends into Pakistan, India and Nepal. Although the kiang is vulnerable to habitat destruction, enough individuals survive over a wide enough area that the species is not yet under threat.
Humans drove the once numerous quagga (Equus quagga quagga) to extinction in 1883. In appearance, the quagga was similar to the surviving species of zebra; although it had a dun color and lacked stripes on its back. An ongoing project is underway to breed animals genetically and morphologically similar to the quagga from closely related plains zebras, of which the quagga was a subspecies.