Family that includes the modern horses, asses, and zebras (all of which are placed in a single genus Equus), and many extinct forms. Sufficient of these are believed to be ancestral to modern equids for the evolution of the family to have been traced in considerable detail. The horses are first represented in the fossil record by Hyracotherium (‘Eohippus’) which diverged from a condylarth predecessor in the Palaeocene. Numerous evolutionary lines subsequently appeared from this fox-sized prototype. Most of the evolutionary advances occurred in the New World, although the Equidae were to survive the Pleistocene only in the Old World. The conquistadores reintroduced horses to the Americas in the 16th century. The domestic horse (E. caballus) is probably not descended from the only true living wild horse, Przewalski’s horse (E. ferus przewalskii), but from a progenitor closely related to it.