Recommended Equine Photography Lens

The Canon EF 500mm f/4 L IS II USM Lens is an excellent outdoor equine photography lens. I was testing this lens' autofocus capabilities when this picture was taken. I used a low-to-the-ground shooting position to keep the rider's head above the skyline. I will note that I was not completely comfortable (psychologically) shooting close to the ground with a 1,000 lb animal bearing down on me at high speed.