Apparently, they were using actual unbroke horses for a rodeo-like 'catch' the horse scene in an arena... It's as dangerous, I suppose as the people who do this in real life every day. It adds an element of realism to the movie to use actual unbroke mustangs, but it's possible the same accident might have happened even with a trained horse re-enacting the same scene. I know that leaving a leadrope on one of these mustangs is a pretty common method by people until they are easy to catch. It has risks... but people are always taking some risk every time they try to halter a large animal and get them to do our bidding.
I heard on the radio that they were rodeo horses from Montana and that the owner of the horses {probably a little angry} is having the rest of his horses shipped back home to Montana immediately. No more scenes with his horses for this movie.