There were no horses in America, there were evolutionary ancestors of horses that would not be able to fulfill any horse role.
Just like zebras are not horses and wolves not dogs. They would obviously not be owned by Native Americans nor would the Native Americans have a remarkable body of knowledge about them (like they developed with actual horses).
Horses were bred to be big and strong enough in Central Asia.
There were no horses in America, there were evolutionary ancestors of horses that would not be able to fulfill any horse role.
Just like zebras are not horses and wolves not dogs. They would obviously not be owned by Native Americans nor would the Native Americans have a remarkable body of knowledge about them (like they developed with actual horses).
Horses were bred to be big and strong enough in Central Asia.