Many Olympians rely on other athletes for their success. In basketball, soccer, volleyball, water polo, crew and relays, you need teammates or partners competing at their mental and physical peak for any hope of winning a medal.
But at least those teammates are human. Having a horse for an Olympic partner is somewhat more involved. Since equestrian requires experience on so many different levels, riders reach (and remain at) the elite level far later.
Be sure to check out the full video here: http://espn.go.com/oly/feature/video/_/page/londonhopefuls/game-changers , and hear from one of the best riders in the world!