From the British documentary series Inside Nature's Giants, the Thoroughbred horse is examined to show its strengths and weaknesses, and just how so large an animal is capable of such great speed. (Be forewarned that it includes dissection of a horse, which some may not be able to watch, but which is helpful in understanding just how amazing these animals are, and why some are simply faster than others.)
https://www.youtube.com/watch?v=WHlqZ6VZvKE