I wanted to add that the polo pony I rode most often was nicknamed "The Animal" and considered one of the hardest horses to ride in our college string. Now, I am no exceptional rider, believe me, and she was hands down the most sensitive and just plain "easy" horse I've ever ridden. I've never been very good at dressage, but I could make that horse go from collected to extended trot and move laterally with just my seat - I have never had the ability to do that on another horse. I credit her training and her personality totally for making me look good
And she knew the rules of the game far better than I did - I just had to look at the ball or another horse and she knew what to do.
I also know a pony who jumped a 4' arena door accidentally. The exercise was to stop as close to the wall as possible, so his rider sat down and rode him to the base of the wall and the horse jumped it easily - the first jump of his life.
They're really amazing horses and I don't think they get nearly enough press or credit.