Most important of all is the Orohippus, from the Eocene.
"The Old Riddle and the Newest Answer"
John Gerard
"But the most important discovery of all is the Orohippus which comes from the Eocene formation, and is the oldest member of the equine series yet known.
"Thomas Henry Huxley; A Sketch Of His Life And Work"
P. Chalmers Mitchell
In the first of those ancestors-Orohippus-there were four digits; all save the first, the thumb, being then developed.
"Artistic Anatomy of Animals"
Édouard Cuyer