How were horses brought to the us?

Horses were in the American continents for a long time, when for an unknown reason, they went extinct. ??

Horses then were reintroduced to the America's when the Conquistadors from Spain arrived to explore the new world. (only the Spanish horses)

The American mustangs/qh's have always been around they have never gone extinct; hence 'American' in their names

they also migrated across the ice bridge as the cave men, and so many other animals did looking for food.

here is a link: http://www.madsci.org/posts/archives/2000-06/960351408.Zo.r.html

to the answer too