History of Trick Riding
Trick Riding traces its roots to the Romans, who stood on their horses to race, and the Russian Cossacks, an army group who would do stunts such as hanging off the side of the horse.  Trick riding in America began in the early 1900's as a rodeo contest event.  Cowboys and cowgirls entered the trick riding event--pushing their bodies, strength, equipment, and horses to the limit as they sought to outdo one another in terms of difficulty, speed, and danger.  When the competition became so intense that the danger became out of control, trick riding evolved into the specialized, contract act that we now know as modern trick riding.  Modern trick riding focueses much more on poise, grace, balance and style than did early trick riding. 
The Horses
Trick riding demands steady, trustwothy horses with a good disposition.  The horse must accept its rider moving all around its body without fear.  The horse's instinct is often to stop when it feels the rider moving about.  The trick horse must be trained to understand that this is okay.  The rider does not use the reins to guide the horse, so the horse must be very well trained.  It must be taught to run fast and true without spooking or swerving.  Many trick riders prefer geldings.