To give 'em a niche, we might consider giving them a special ability similar to the Fleet of Foot feat...
Benefit: When running or charging, you can make a single direction change of 90 degrees or less. You can't use this feat in medium or heavy armor, or if you're carrying a medium or heavier load. If you are charging, you must move in a straight line for 10 feet (2 squares) after the turn to maintain the charge.
Bison are CR 2, but these guys can get in two nasty hits a round, as opposed to the bison's 1. So let's either make them CR 3 or drop the damage on those hooves to 1d4.