How intelligent are horses?

The horse must be a pretty remarkable animal. If you have one, or are really familiar with them, what stories can you relate that illustrate just how smart they can be?

This question arose from thinking about how quickly horses that escaped from Spanish Conquistadors in Central America in the 1500s migrated up through Mexico into Texas and many parts of the the North American South and North-West. Obviously the Plains Indians recognized that this was a far more intelligent and domesticable animal than the bison, deer, caribou or elk. They didn’t see the horse as meat, they saw it as a valuable animal they could domesticate.

By the 1800s when American settlers began to push into the Western territories, the Plains Indians were incredibly skillful horsemen. How did the Indians recognize that the horse was far more valuable as an ally and friend than as prey? How did that symbiotic relationship form so quickly? How much did plains Indians treat horses like intelligent animals and pets?