Where does the horse originally come from?

The horse evolved from a group of small leaf-eating mammals that emerged in North America about 50 million years ago. 
Eohippus, also called the “dawn horse,” was a thin-limbed animal that foraged in forests and was about the size of a fox. It walked on toes that had small hooves: four on the front legs, three on the hind legs. 
Over millions of years, through natural selection, the dawn horse evolved into larger, a grass-grazing animal with long and slender legs, each featuring a single hoof. Some of the oldest fossils have been found in Wyoming and Idaho.

While horses originated in North America they became extinct on the continent about 10,000 years ago. Long before that happened, however, herds migrated and spread all over the world where they continued to evolve.

Horses were reintroduced to North America by Europeans, including Spanish Conquistadors, in the 15th Century. 

In a throwback to the origins of the horse, today’s animals have a patch of hardened skin on the inside of each foreleg. Some scientists theorize these callosities, called chestnuts, are remnants of the multi-toed Eohippus, an extra toe that was reduced to a diminished form through evolution. 
Archaeologists believe humans began taming, raising, and breeding horses at least 6,000 years ago, possibly starting in Eastern Europe in the steppe lands north of the Black Sea, which includes Ukraine and Kazakhstan. It’s very probable, however, that domestication began in multiple areas during this time.