The horse family (Equidae) today is quite small. All horse breeds, from slim thoroughbred racehorses to stocky plow horses to tiny ponies, belong to a single species, Equus caballus. What's more, all surviving branches of the horse family tree are also members of this same genus Equus, which now consists of only seven living species. Other equids include donkeys, asses, and zebras.

Source: American Museum of Natural History