Trying to teach a horse its leads can be frustrating whether you're training a greenie or reschooling a horse with some bad habits. Before you lose your temper or your sanity, realize first that every horse is a little one-sided. Just like humans, they prefer a left or right-handed approach to life. You can discover which lead your horse prefers when it's running loose or on the lunge line. Your job as a rider is to use your aids and a good sense of timing to set your horse up to strike off on the correct lead.
You can begin some of your work on the lunge line. You’ll need a snaffle bridle, a surcingle with side dee rings, and a pair of side reins. You’ll use the side reins to help your horse bend in the direction of his circle, keeping him balanced. This will encourage him to take the correct lead.
Let’s say your horse has trouble taking his right lead. Gradually adjust the side reins so that the right rein is several inches shorter than the left. Your horse should tip his nose and bend his neck to the right as he yields to the slight pressure. Now begin lunging him to the right at the trot. As you ask him to canter, time your request to coincide when your horse appears relaxed and primed to step into his right lead. It may take you a few tries to educate your eye, but you should be able to tell when he’s prepared to take the right lead. If he still takes the left lead, calmly bring him back to the trot and try again. When he is correct, praise him and urge him to canter several times around so he strengthens his muscles and improves his balance.
Now transfer the same methods to under saddle work. Begin trotting in a large circle to the right, asking your horse to bend in the direction he’s traveling. Sit back in the saddle and use your seat and legs to drive your horse forward. As you keep him bent to the right, strongly push with a definite outside leg as you ask for the canter. Is he on the right lead? If so, finish your circle then continue around your arena, praising your horse. If he’s wrong, gently come back to the trot, regroup and rebalance, and try again.
If you continue to have problems, there are two possibilities. One is that your horse has not been schooled to move away from your leg. As you ask for a canter depart with your outside leg, you are in essence asking your horse to displace his haunches ever so slightly to the inside at the same instance you’re driving him into the canter. This response to your outside leg is what pushes him onto the correct lead. If your horse resists, or ignores, moving away from leg pressure, you cannot hope to get the correct lead. The second possibility is that your horse has a soundness problem. A painful back or sore hocks can make a horse hesitant to engage its hindquarters. Have your veterinarian examine your horse to rule out any lameness issues.
Finally, before your horse begins to really object to taking his leads by bucking, tossing his head or simply refusing to canter, consult with a professional trainer. You may find that you are restricting your horse in the mouth, or shifting your weight in the saddle, both of which may throw your horse off balance. With some guidance, you and your horse will be striking off on the right hoof in just a short time!