In body form all equids are quite similar. They are specialized for running and for grazing on siliceous grasses, which rapidly wear down the teeth. Species can be differentiated, however, on the basis of qualitative and quantitative characteristics of the skull, tooth morphology and length and robustness of the metapodials (foot bones). There are also variations in size, color, color patterning, hoof size and shape, mane length, presence or absence of a forelock, tail hair length and distribution, number and size of chestnuts and vocalizations
One reason all equids are vulnerable in the face of hunting pressures and habitat loss is that they reproduce slowly. Gestation lasts eleven to thirteen months, depending upon the species, and almost always only one foal I dropped each season. Most researchers, however, report that mares foal only every other year. Sexual maturity occurs at age two in females and age three to five in males. Life span can extend twenty or more years
Close relatives of the horse persist in the wild today in several parts of the Old World. Taxonomic interpretations vary, but most experts place them all in the genus Equus. The handsome striped equids of Africa fall into three distinct species: Grevy's zebra (Equus grevyi), the plains or Burchell's zebra (Equus burchelli =Equus quagga), and the mountain zebra (Equus zebra). Of these, the Grevy's zebra is most distinct. Taxonomically it is placed in the subgenus Dolichohippus, whereas the plains and mountain zebras are placed in the subgenus Hippotigris. The continued survival of most of these fascinating and spectacular species is threatened. Despite the establishment of sanctuaries, many populations are faced with diminishing ranges and decreasing numbers.