Pleistocene horse fossils have been assigned to a multitude of species, with over 50 species of equines described from the Pleistocene of North America alone, although the taxonomic validity of most of these has been called into question. Recent genetic work on fossils has found evidence for only three genetically divergent equid lineages in Pleistocene North and South America. These results suggest all North American fossils of caballine-type horses (which also include the domesticated horse and Przewalski's horse of Europe and Asia), as well as South American fossils traditionally placed in the subgenus "E. (Amerhippus)" belong to the same species: "E. ferus". Remains attributed to a variety of species and lumped as New World stilt-legged horses (including "H. francisci", "E. tau", "E. quinni" and potentially North American Pleistocene fossils previously attributed to "E. cf. hemiones", and "E. (Asinus)" cf. "kiang") probably all belong to a second species endemic to North America, which despite a superficial resemblance to species in the subgenus "E. (Asinus)" (and hence occasionally referred to as North American ass) is closely related to "E. ferus". Surprisingly, the third species, endemic to South America and traditionally referred to as "Hippidion", originally believed to be descended from "Pliohippus", was shown to be a third species in the genus "Equus", closely related to the New World stilt-legged horse. The temporal and regional variation in body size and morphological features within each lineage indicates extraordinary intraspecific plasticity. Such environment-driven adaptative changes would explain why the taxonomic diversity of Pleistocene equids has been overestimated on morphoanatomical grounds.