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.
Well, if he has rodeo horses, broncs probably for a job like this, this is probably not the first severe injury he's had. His horses burn out, get injured and are shipped off for dog food doing their day job.
Yes, they decided not to used untrained horses for the rest of this shoot, but I wouldn't go putting a bunch feel-good motives on the part of the owner of the horse that was killed. This isn't about any eeeevil film makers who mistreated his animals in some covert way, he knew what his horses would be used for and the risks involved.