Horse evolution
From EvoWiki
The evolution of equids from Hyracotherium to Equus is excellently illustrated by an abundant fossil record. One can easily see how a more-or-less generalized mammal like Hyracotherium became the horses, donkeys, and zebras of today. It is thought to originate in the American continent, but was transported to the old world either by boat or landbridge during the ice ages. The horse went extinct in America for a while, possible due to humans consuming them, or due to environmental changes, and was reintroduced by the spanish in the 1500's.
External Links
Some excellent talk.origins articles on horse evolution.
PZ Myers's Pharyngula weblog article, discussing recent data.
