This horse sport is something like Polo meets Basketball - riders handle the ball (which is designed for easy gripping), pass it by throwing, and score by throwing it into a high net. While riding. Seems to have a fairly passionate following in France, in particular, although it is played internationally, and as of 2006 the US had a team capable of international competition. It was invented in France in the 1930s, but didn't gain widespread popularity until the 1990s. Here's the official site, for all the details. The site for the US Horseball Federation is here.
Check out the excitement at the match show here: