I am wondering if a faster shutter speed combined with a wider aperture, producing a more narrow DOF might not get a different, yet similar product with the horse and rider in focus and the background OOF. While this freezes the horse, it shows motion by capturing the horse's hooves off the ground. Here is a series of three shots I did at a polo match as an example of what I mean. These were shot in burst mode using a Canon 300mm f/4L IS lens using AV at f/4 and ISO 200 using servo AF with AF point expansion. The f/4 aperture allowed me to get the entire horse in focus because of the distance at which I was shooting while blurring the background and, at my maximum lens aperture, the ISO 200 produces a fast enough shutter speed to freeze action...
I do like shot number one because of the position of mallet and ball. However, I tend to like the second shot best because of the cleaner background as well as the positioning of the pony's hooves. The third shot shows how wonderfully athletic these polo ponies are...
A disadvantage of shooting Polo is that the players and ponies do not ride in a circle. They are all over the place on a tremendously large field. However, an advantage of shooting polo is that a match will allow many opportunities for photos with the the ponies and riders charging of in different directions.