Phylogeny is the history of the evolution of a species or group, especially in reference to lines of descent and relationships among broad groups of organisms. This is done using fossils available and establishing common links, their divergence point in time and understanding changes that occurred with time for adaptation.  The history of the horse covers a period of 60,000,000 years.  The first horse appeared in the Eocene period about 6 million years ago as a small horse-like animal which was about 12' high and had four toes and remain of 5th toe. That extinct animal is named Eohippus.  The second stage is represented by Orohippus which were a little more advanced than Eohippus, having lost the remains of 5th digit of the foot. The next stage is represented by the three-toed horse called mesohippus which made its appearance in the Oligocene period. The height of the animal was double that of an eohippus. The teeth had also become more complex. Mesohippus was later replaced by Merychippus in the Miocene period. Merychippus represented a transitional stage between modem horse and Mesohippus. Pliohippus, the first single-toed horse, appeared in Pliocene. It was about 40 inches in height. The modem horse Equus first appeared in the upper Pleistocene and now represents the final stage of evolution of the horse.