Hyracotherium
Hyracotherium was an ancient horse species that lived during the Cenozoic Era, specifically just before and during the Eocene Epoch. Diminutive compared to modern Equus equus, this animal was also comparatively primitive. Terror Bird genus, Phorusrhacos are depicted eating one in the book with a still image of two tearing the animal apart with their hooked beaks (though this is in itself an erroneous concept, as the Hyracotherium died out a very long time before even the ancient Phorusrhacid Terror Bird line evolved - though perhaps the graphic was intended to be a small horse of the time)