“The earliest-known horse Sifrhippus started out this period as a bit of a pipsqueak: The animals weighed only about 12 pounds (5.6 kilograms) on average, about the size of a miniature schnauzer.”
Liberal solution- Shrink the size of human beings in order to be able ride a 12 lb. horse.
Yes, that was the chain of events as I have read. They had three toes at that time.
The horses had to get small to fit into the cars they needed to drive in order to stop global warming. This all makes more sense now!/s/