When you're riding a horse and scream "Whoa!" to make it stop, you're pulling on the reins, which are attached to a thing called the bridle, the buckled straps around a horse's head that help you control its movements.
The word bridle comes from the Old English bridel meaning "rein, curb, restraint," which is precisely what the purpose of a bridle is — to help restrain a horse's movements when necessary. You can bridle a horse, which is the act of putting a bridle on it. If you yank too hard on the reins, your horse might bridle, or take offense to your aggressiveness, just like you might bridle if someone insulted you.