An Ancient History
The evolution of the horse is well documented by a lengthy fossil record extending over 55 million years. During this time, episodes of relatively rapid modification were interspersed with long periods of little change. And it is clear that horses adapted to many different environments and pressures. Careful dating and analysis of fossils shows a clear succession of forms, from the dog-sized Hyracotherium to Equus, the modern horse.
Hyracotherium, Merychippus, and Equus represent stages in the evolutionary path to living horses, a sequence that shows species becoming larger through time. But not all ancient horses evolved into larger forms; members of some extinct groups were actually smaller than their ancestors. Indeed, there are no universal trends in evolution. Over time, a population can demonstrate a net trend in one direction, but over smaller intervals, some subgroups might evolve differently.