The horse thing IS interesting.
The most widespread theory is that horses evolved in the Americas and migrated west to Asia. They then went extinct in the Americas (like the mammoth) ~10,000 years ago before being re-introduced by the Spaniards in the early 16th Century.
But apparently there are some other ideas out there that I have never heard of, contending that the North American horse didn't go extinct. Interesting stuff...
The Aboriginal North American Horse