From my long-time equine companion, Ladybug, I learned horses have a very strict social order based on respect.  I also learned that you are either the boss of them or they are of you...there is no in-between.  They are herd, prey animals that look to the herd leader to tell them what to do.  First we learn their language of body language and then we learn how to become their leader.  Earning their respect and becoming their leader ensures a safe, rewarding relationship.