Horses are big and powerful and can do a lot of damage.
Did you ever hear the story about how Will Rogers broke into show biz big time? He was a participant in a vaudeville rodeo [Madison Square Gardens], and a steer jumped the railing and headed up into the stands. Will jumped the railing on his horse, lassoed the steer and calmed it. The rest is history.
Pants were not invented until the horse was domesticated. They (pants) were necessary to straddle the horses back.