Horses are clear, ears back and they are not happy, flair of nostril even more unhappy, add a buck, a kick into the air or at each other and we've got a problem.
Generally it is over their space, their territory and their food. It is also used by the lead mare to establish her place in the herd. In the wild the lead mare or stallion needs all to follow and respond when asked because it usually means life or death.
Due to centuries of surviving in the wild, the pastures of today still have horses establishing herd hierarchy.
I love these pictures because I can catch a glimpse of the girls and what they would have looked like centuries ago in the wild.