A sketch showing the supposed evolution of the horse has appeared in nearly every textbook dealing with evolution. Unfortunately, this presentation does not contain the whole truth. In modem decades, the tree of horse evolution has been refuted and abandoned, in professional circles at least.
Dotted lines represent uncertainties in dating.
Most individual categories are known only from their teeth. It is hard to see any evolutionary sequence here. Eohippus had 18 pairs, Orohippus had 15 pairs, Pliohippus jumped to 19 pairs, and the modern horse had 18. Furthermore, the Eohippus, more properly named Hyracotherium, is remarkably similar to the modern hyrax, a rock badger. The San Diego Zoo keeps a colony, where a sign identifies them as similar to the animal, which evolved into the horse and elephant.