Mesophippus is a genus of early species of horse from Early Oligocene. It was the same size as a pony, but looked a bit like a horse. Like all horses they lived in herds and when they sensed danger they ran. Mostly herbivores, they ate low growing plants.
In the Series 3 episode "The Big, the Bad, and the Ugly," a herd of Mesophippus were brought to the park from Late Oligocene North America, 24 million years ago. They reside in the Giraffe Enclosure.