From Nat Geo:
Known today as Hyracotherium ("hyrax-like beast"), Eohippus ("dawn horse") was the original name of the first complete skeleton of this primitive, foxlike horse, discovered in the southern U.S. in 1867.Why it matters: When Darwin went public with his theory of evolution, there was no hard evidence to show how an existing animal had evolved from prehistoric species—until Hyracotherium, kicked off a series of fossil discoveries depicting the evolution of horses over 55 million years.Paul Sereno of the University of Chicago says Hyracotherium/Eohippus is up there historically with Archaeopteryx. The paleontologist named Eoraptor—the small transitional dinosaur at the root of the dinosaur family tree—with Eohippus in mind.