In paleontology, correctly naming a new genus of extinct animal can often be a long, tortured affair. Eohippus, aka Hyracotherium, is a good case study: this prehistoric horse was first described by the famous 19th-century paleontologist Richard Owen, who mistook it for an ancestor of the hyrax (hence the name he bestowed on it in 1876, Greek for "hyrax-like mammal"). A few decades later, another eminent paleontologist, Othniel C. Marsh, gave a similar skeleton discovered in North America the more memorable name Eohippus ("dawn horse").
Since for a long time Hyracotherium and Eohippus were considered to be identical, the rules of paleontology dictated that we call this mammal by its original name, the one bestowed by Owen. Never mind that Eohippus was the name used in countless encyclopedias, children's books, and TV shows. Now, the weight of opinion is that Hyracotherium and Eohippus were closely related, but not quite identical, the result is that it's once again kosher to refer to the American specimen, at least, as Eohippus. Amusingly, the late evolutionary scientist Stephen Jay Gould railed against the depiction of Eohippus in the popular media as a fox-sized mammal, when in fact it was the size of a deer.
An Ancestor of Modern Horses
There's a similar amount of confusion about whether Eohippus and/or Hyracotherium actually deserve to be called the "first horse." When you go back in the fossil record 50 million years or so, it can be difficult, verging on impossible, to identify the ancestral forms of any given extant species. Today, most paleontologists classify Hyracotherium as a "palaeothere," that is, a perissodactyl (odd-toed ungulate) ancestral to both horses and the giant plant-eating mammals known as brontotheres (typified by Brontotherium, the "thunder beast"). Its close cousin Eohippus, on the other hand, seems to deserve a place more firmly in the equid than the palaeothere family tree, though of course, this is still up for debate!
Whatever you choose to call it, Eohippus was clearly at least partly ancestral to all modern-day horses, as well as to the numerous species of prehistoric horse (like Epihippus and Merychippus) that roamed the North American and Eurasian plains of the Tertiary and Quaternary periods. As with many such evolutionary precursors, Eohippus didn't look much like a horse, with its slender, deerlike, 50-pound body and three- and four-toed feet; also, to judge by the shape of its teeth, Eohippus munched on low-lying leaves rather than grass. (In the early Eocene epoch, when Eohippus lived, grasses had yet to spread across the North American plains, which spurred the evolution of grass-eating equids.)
Facts About Eohippus
Eohippus (Greek for "dawn horse"), pronounced EE-oh-HIP-us; also known as Hyracotherium (Greek for "hyrax-like beast"), pronounced HIGH-rack-oh-THEE-ree-um
Woodlands of North America and Western Europe
Early-Middle Eocene (55-45 million years ago)
Size and Weight:
About two feet high and 50 pounds
Small size; four-toed front and three-toed back feet