Bronco riding came from what cowboys did on the ranch when they got new horses.  This changed how other people would ride.  Then, the cowboys got together every year and rode.  This was the first action which started bronc riding and also calf roping. Cowboys had to rope the calf and brand them.  Branding means they would know which cow came from which ranch.
Then, later, cowboys developed bull riding. That is the most dangerous.  37%, or 37 percent, of all injuries come from bull riding.  Imagine yourself riding a bronco....plunging up.  You have to hold onto a single rope. You can't touch yourself, you can't touch the horse.  Just only the rope....WOW!!!

The video below, cowboy riding a bucking horse,  was video taped from an original video at the National Cowboy Museum.  This is called bronco riding.

The video below, bull riding, was video taped from an original video seen at the National Cowboy Museum.  This is called bull riding.  Bull riding is the most dangerous!