When a horse is a confirmed rearer, tying it's head to a hind leg is an old method. When the horse attempts to rear, the hind leg is pulled, not enough that the horse falls down, but just that little bit that it rattles the horse. I have found that when a horse does something to itself the lesson is better learned. One old German trainer would set up a well bedded area and place a hobble on one leg of the horse to which there was a long rope attached. After studying the horse he'd figure out how to cause the horse to rear, When it did, just as the fronts were coming down he'd pull on it which of course frightened the horse. His front feet had always been under him for a safe landing and suddenly one wasn't. Actually he'd have his feet under him but the landing was off balance. That was often all it took to stop that bad habit.