Science, Tech, Math › Animals & Nature 10 Prehistoric Horses Everyone Should Know Share Flipboard Email Print Animals & Nature Dinosaurs Prehistoric Mammals Basics Paleontologists Carnivores Dinosaurs & Birds Herbivores Marine Reptiles Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Marine Life Forestry Evolution View More By Bob Strauss Science Writer B.S., Cornell University Bob Strauss is a science writer and the author of several books, including "The Big Book of What, How and Why" and "A Field Guide to the Dinosaurs of North America." our editorial process Bob Strauss Updated February 08, 2019 01 of 11 Are You Familiar With These 10 Prehistoric Horses? Yinan Chen / Wikimedia Commons / CC The ancestral horses of the Cenozoic Era are a case study in adaptation: as primitive grasses slowly, over the course of tens of millions of years, covered the North American plains, so did odd-toed ungulates like Epihippus and Miohippus evolve both to nibble on this tasty greenery and traverse it swiftly with their long legs. Here are ten important prehistoric equines without which there'd be no such thing as a modern Thoroughbred. 02 of 11 Hyracotherium (50 Million Years Ago) Jonathan Chen / Wikimedia Commons / CCA-SA 4.0 If the name Hyracotherium ("hyrax beast") sounds unfamiliar, that's because this ancestral equine used to be known as Eohippus ("dawn horse"). Whatever you choose to call it, this famously tiny odd-toed ungulate—only about two feet high at the shoulder and 50 pounds—is the earliest identified horse ancestor, an inoffensive, deer-like mammal that traveled the plains of early Eocene Europe and North America. Hyracotherium possessed four toes on its front feet and three on its rear feet, a long way from the single, enlarged toes of modern horses. 03 of 11 Orohippus (45 Million Years Ago) Daderot / Wikimedia Commons / [CC0] Advance Hyracotherium by a few million years, and you'll wind up with Orohippus: a comparably sized equid possessing a more elongated snout, tougher molars, and slightly enlarged middle toes on its front and hind feet (an adumbration of the single toes of modern horses). Some paleontologist "synonymize" Orohippus with the even more obscure Protorohippus; in any case, this ungulate's name (Greek for "mountain horse") is inappropriate, as it flourished on the North American plains. 04 of 11 Mesohippus (40 Million Years Ago) David Starner / Wikimedia Commons / CCA-3.0 Mesohippus ("middle horse") represents the next step in the evolutionary trend kicked off by Hyracotherium and continued by Orohippus. This late Eocene horse was slightly bigger than its forebears—about 75 pounds—with long legs, a narrow skull, a relatively large brain, and widely spaced, distinctly horselike eyes. Most important, the front limbs of Mesohippus had three, rather than four, digits, and this horse balanced itself mainly (but not exclusively) on its enlarged middle toes. 05 of 11 Miohippus (35 Million Years Ago) mark6mauno / Wikimedia Commons / CCA-SA 2.0 A few million years after Mesohippus comes Miohippus: a slightly larger (100 pound) equid that achieved a widespread distribution across the North American plains during the late Eocene epoch. In Miohippus, we see the continued lengthening of the classic equine skull, as well as longer limbs that allowed this ungulute to thrive in both plains and woodlands (depending on the species). By the way, the name Miohippus ("Miocene horse") is a flat-out mistake; this equid lived more than 20 million years before the Miocene epoch! 06 of 11 Epihippus (30 Million Years Ago) Ghedoghedo / Wikimedia Commons / CCA-SA 3.0 At a certain height of the horse evolutionary tree, it can be hard to keep track of all those "-hippos" and "-hippi." Ephippus seems to have been a direct descendant not of Mesohippus and Miohippus, but of the even earlier Orohippus. This "marginal horse" (the Greek translation of its name) continued the Eocene trend of enlarged middle toes, and its skull was equipped with ten grinding molars. Crucially, unlike its predecessors, Epihippus seems to have thrived in lush meadows, rather than forests or woodlands. 07 of 11 Parahippus (20 Million Years Ago) Claire H. / Wikimedia Commons / CCA-SA 2.0 Just as Epihippus represented an "improved" version of the earlier Orohippus, so Parahippus ("almost horse") represented an "improved" version of the earlier Miohippus. The first horse listed here to achieve a respectable size (about five feet tall at the shoulder and 500 pounds), Parahippus had comparably longer legs with larger middle toes (the outer toes of ancestral horses were nearly vestigial by this stretch of the Miocene epoch), and its teeth were shaped perfectly to handle the tough grasses of its North American habitat. 08 of 11 Merychippus (15 Million Years Ago) Momotarou2012 / Wikimedia Commons / CCA-SA 3.0 Six feet tall at the shoulder and 1,000 pounds, Merychippus cut a reasonably horselike profile, if you're willing to ignore the small toes surrounding its enlarged middle hooves. Most important from the perspective of equine evolution, Merychippus is the first known horse to have grazed exclusvely on grass, and so successfully did it adapt to its North American habitat that all subsequent horses are believed to have been its descendants. (Yet another misnomer here: this "ruminant horse" wasn't a true ruminant, an honor reserved for ungulates, like cows, equipped with extra stomachs). 09 of 11 Hipparion (10 Million Years Ago) PePeEfe / Wikimedia Commons / CC By 4.0 Represented by a dozen separate species, Hipparion ("like a horse") was hands-down the most successful equid of the latter Cenozoic Era, populating the grassy plains not only of North America but also Europe and Africa. This direct descendant of Merychippus was slightly smaller—no species are known to have exceeded 500 pounds—and it still retained those giveaway vestigial toes surrounding its hooves. To judge by this equid's preserved footprints, Hipparion not only looked like a modern horse—it ran like a modern horse as well! 10 of 11 Pliohippus (5 Million Years Ago) Ghedoghedo / Wikimedia Commons / CC by 4.0 Pliohippus is the bad apple on the equine evolutionary tree: there's reason to believe that this otherwise horse-like ungulate was not directly ancestral to genus Equus, but represented a side branch in evolution. Specifically, this "Pliocene horse" had deep impressions in its skull, not seen in any other equid genus, and its teeth were curved rather than straight. Otherwise, though, the long-legged, half-ton Pliohippus looked and behaved much like the other ancestral horses on this list, subsisting like them on an exclusive diet of grass. 11 of 11 Hippidion (2 Million Years Ago) Ghedoghedo / Wikimedia Commons / CC by 4.0 Finally, we come to the last "hippo": the donkey-sized Hippidion of the Pleistocene epoch, one of the few ancestral horses known to have colonized South America (by way of the recently unsubmerged Central American isthmus). Ironically, in light of the tens of millions of years they spent evolving there, Hippidion and its northern relatives went extinct in the Americas shortly after the last Ice Age; it remained for European settlers to reintroduce the horse into the New World in the 16th century AD.