There are three extant species.

Zebras evolved among the Old World horses within the last 4 million years. Grévy's zebras (and perhaps also Mountain Zebras) are, together with asses and donkeys, in a separate lineage from other zebra lineages.[2] This means either that striped equids evolved more than once, or that common ancestors of zebras and asses were striped and only zebras retained the stripes. Extensive stripes are posited to have been of little use to equids that live in low densities in deserts (like asses and some horses) or ones that live in colder climates with shaggy coats and annual shading (like some horses).[3]
Fossils of an ancient equid were discovered in the Hagerman Fossil Beds National Monument in Hagerman, Idaho. It was named the Hagerman horse with a scientific name of Equus simplicidens. It is believed to have been similar to the Grévy's zebra.[4] The animals had stocky zebra-like bodies and short, narrow, donkey-like skulls.[5] Grévy's zebra also has a donkeylike skull. The Hagerman horse is also called the American zebra or Hagerman zebra. Another equid known as Equus occidentalis is called the Western Horse or Western Quagga quite possibly a subspecies of plains zebra.
Classification


Zebras in Botswana
There are three extant species. Collectively, two of the species have eight subspecies (seven extant). Zebra populations are diverse, and the relationships between, and the taxonomic status of, several of the subspecies are not well known.
Genus: Equus
Subgenus: Hippotigris
Plains zebra, Equus quagga
Quagga, Equus quagga quagga (extinct)
Burchell's zebra, Equus quagga burchellii (includes Damara Zebra)
Grant's zebra, Equus quagga boehmi
Selous' zebra, Equus quagga borensis
Chapman's zebra, Equus quagga chapmani
Crawshay's zebra, Equus quagga crawshayi
Mountain zebra, Equus zebra
Cape mountain zebra, Equus zebra zebra
Hartmann's mountain zebra, Equus zebra hartmannae
Subgenus: Dolichohippus
Grévy's zebra, Equus grevyi

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