While the domestic horse and donkey (along with their feral descendants) exist worldwide, wild equine populations are limited to Africa and Asia.

[9] The first equids were small, dog-sized mammals (e.g. Eohippus) adapted for browsing on shrubs during the Eocene, around 54 million years ago (Mya).

[11] The genus Equus, which includes all extant equines, is believed to have evolved from Dinohippus, via the intermediate form Plesippus.

The genus appears to have spread quickly into the Old World, with the similarly aged E. livenzovensis documented from western Europe and Russia.

[11] Molecular phylogenies indicate that the most recent common ancestor of all modern equines (members of the genus Equus) lived ~5.6 (3.9-7.8) Mya.

[8][13] Of the extant equine species, the lineage of the asses may have diverged first,[9][contradictory] possibly as soon as Equus reached the Old World.

[8] Members of the subgenus Sussemionus were abundant during the Early and Middle Pleistocene of North America and Afro-Eurasia,[14] but only a single species, Equus ovodovi survived into the Late Pleistocene and Holocene in south Siberia and China, with the youngest remains from China dating to around 3500 BP (1500 BC), during the Shang dynasty.

[15][16] Genetic data from E. ovodovi has placed the Sussemionus lineage as closer to zebras and asses than to caballine horses.

DNA studies on American horse remains found frozen into permafrost have shown that several of the supposed American species, as well as the European Equus ferus, are actually a single highly-variable widespread species, as if the evolutionary process of speciation was persistently being frustrated by large herds moving long distances and mixing.

[32] Ancient DNA identifies the Bronze Age kunga as a cross between the Syrian wild ass and the donkey.

Grévy's zebra is the largest wild species, standing up to 13.2 hands (54 inches, 137 cm) and weighing up to 405 kg (890 lb).

Their dentition is adapted for grazing; they have large incisors that clip grass blades and highly crowned, ridged molars well suited for grinding.

[39] The purpose of the bold black-and-white striping of zebras has been a subject of debate among biologists for over a century, but 2014 evidence supports the theory that they are a form of protection from biting flies.

These insects appear to be less attracted to striped coats, and compared to other wild equines, zebras live in areas with the highest fly activity.

[43] They prefer to eat grasses and sedges, but they, particularly asses, may also consume bark, leaves, buds, fruits, and roots if their favored foods are scarce.

Young of both sexes leave their natal groups as they mature; females are usually abducted by outside males to be included as permanent members of their harems.

These species live in habitats with sparser resources and standing water, and grazing areas may be separated.

Equines also communicate with visual displays, and the flexibility of their lips allows them to make complex facial expressions.

Flattened ears, bared teeth, and abrupt movement of the heads may be used as threatening gestures, particularly among stallions.

[46] Estrus in female equines lasts 5–10 days; physical signs include frequent urination, flowing muscus, and swollen, everted labia.

In addition, estrous females will stand with their hind legs spread and raise their tails when in the presence of a male.

Length of gestation varies by species; it is roughly 11–13 months, and most mares come into estrus again within a few days after foaling, depending on conditions.

[9] Species in arid habitats, like Grévy's zebra, have longer nursing intervals and do not drink water until they are three months old.

[47] Among harem-holding species, foals are cared for mostly by their mothers, but if threatened by predators, the entire group works together to protect all the young.

[36] In territory-holding species, mothers may gather into small groups and leave their young in "kindergartens" under the guard of a territorial male while searching for water.

[45] The earliest archaeological evidence for the domestication of the horse comes from sites in Ukraine and Kazakhstan, dating to around 4000–3500 BCE.

[50] The most recent, but most irrefutable, evidence of domestication comes from sites where horse remains were buried with chariots in graves of the Sintashta and Petrovka cultures c. 2100 BCE.

[52] Archaeological, biogeographical, and linguistic evidence suggests that the donkey was first domesticated by nomadic pastoral people in North Africa over 5,000 years ago.

[59] Attempts to domesticate zebras were largely unsuccessful, though Walter Rothschild trained some to draw a carriage in England.

[68] At present, certain free-roaming horses and burros have federal protection as "living symbols of the historic and pioneer spirit of the West" under the Wild and Free-Roaming Horses and Burros Act of 1971,[69] and in Kleppe v. New Mexico, the United States Supreme Court ruled that the animals so designated were, as a matter of law, wildlife.