With a large herd of 100 mustangs gathered around a waterhole, the wild stallions will enforce the social hierarchy or "pecking order". The dominant horses are the first ones that get to drink. If a lesser horse tries to "cut in", there will be trouble.

The above image is one of six frames shot at high speed. You can see the stallion on the left forcefully shove the other horse into the ground. I was amazed that the horse that was bent over backwards and slammed into the dust was not apparently injured. Notice also that the some of the other horses are watching with interest, like spectators observing a fight behind the gym in Junior High school.