On the left are the front feet of five Miocene horses from the Ashfall site, in contrast to a modern horse
foot on the right. The first three are typical three-toed horses with well-developed side hooves extending down each side
of the main hoof. Three-toed horses were probably good at cornering on the run and might have spent considerable time
in woody or marshy areas. The next two horses functione like single-toed horses, even though the fourth still had
shriveled side hooves. Like modern horses, these ancient horses probably excelled at flat-out running on open grassland.

You can see an animation of the migration of the horse here.