Consider what natural selection is:  the favorable selection by nature for a species of organism to survive and establish itself on the Earth because it possessed a set of characteristics that made it better able to compete with the other organisms in existence at that time.  This hyracotherium, ancestor to the modern horse, was at best, the size of a medium sized dog, about two feet in length and a little over a foot at the shoulder.  It had pads, like a dog, on the bottoms of its toes, four toes on the front and three on the back.  Whatever environmental circumstances that existed must have shifted.  This caused this particular organism to cease to be able to compete in the struggle for survival, and it became extinct, while another relative better suited to exist at that time took over the dominant role.  Whatever the reason was, it was sufficent to remove hyracotherium from the ranks of the existing and replace it with a "new and improved" version.