Size and Weight:
In paleontology, naming new genera of extinct animals can often be a long, tortured affair. 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, Greek for "hyrax-like mammal"). A few decades later, another famous paleontologist, Othniel C. Marsh, gave a similar skeleton the more appropriate name Eohippus ("dawn horse"), but since the two beasts turned out to be identical, by the rules the name has reverted to the earlier, mistaken one.
Whatever you call it, Hyracotherium (or Eohippus) was clearly ancestral to all modern-day horses, as well as 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, Hyracotherium didn't look much like a horse, with its slender, deerlike body and three- and four-toed feet; also, judging by the shape of its teeth, Hyracotherium munched on low-lying leaves rather than grass.