Short answers, pulled from the story.
The first members of the Equidae family appeared 54 million years ago during the early Eocene epoch. These early ancestors roamed the soft forests of North America and were no larger than a modern fox. They possessed brains complex enough to suggest alertness and intelligence despite their small size.
As forests gave way to open grasslands, the number of toes on Equidae feet reduced to a single central third toe for running. Their teeth evolved to grind tough fibrous grasses rather than just shearing soft leaves. The digestive system also changed to become hindgut fermenters capable of processing large quantities of low-quality food.
Genera such as Merychippus and Hipparion existed during the Miocene epoch. These species began to resemble the modern horse with long legs and sturdy builds designed for endurance. The three-toed Hipparion coexisted with single-toed lineages during this period of explosive diversity.
By the early Pleistocene epoch, the vast majority of equid species had vanished leaving only the genus Equus to survive. This extinction was likely driven by a combination of climate change and competition from other herbivores. The genus Equus originated in North America and spread rapidly across the globe to give rise to modern horses donkeys and zebras.
There are seven extant species within the genus Equus today. These species include the domestic horse the wild Przewalski's horse the donkey and various species of zebra. The genus Equus contains twenty-two species in total but only seven remain alive.