A racehorse, coming out of the starting gate, is supposed to ALWAYS start on the right lead (in North America), the OUTSIDE lead. They expect and are trained to switch to the inside lead when they get to the first turn. Then switch back to the outside lead for the straight of way, then back onto the inside lead for the next turn. When galloping for exercise in the morning, (if they have a thoughtful gallop rider) they are ALWAYS asked to start on the outside lead, since they come onto the track on the straight of way. In starting gate training, they are ALWAYS asked to start on the right (outside) lead. A well trained racehorse only has one lead transition, the right one. When a horse has been a racehorse for a long time, it is the sequence of feet that they learn IS the canter depart, that of the right lead, NOT the left lead. Since racehorses are usually ridden without a lot of leg on their sides, they do not know much about leg pressure, or how to respond to leg pressure. And they simply do not have the neural pathways developed to use the sequence of feet (starting with the outside hind foot) to get the left lead. It’s a pretty common occurrence that this is an issue that trainers have to work through when reschooling a racehorse as a riding mount.
Step 1) The horse thinks she’s doing it correctly, just as she has always done, and as she has been taught to do. She is not “misbehaving”. She has to develop a part of her brain to make her “ambidextrous”, so that she can use each side of her body equally in the job description of her new career. This takes time, and understanding on your part, as well as on hers.
Step 2) Develop her responses to your leg cues. Develop her bend. Racehorses don’t have much of this training, they don’t need it to race. Instead of leg cues, they respond to the stick, and the rider’s weight. So much of the training will occur before you ask for the canter.
Step 3) As others have stated, using a pole on the ground can be helpful in getting the sticky lead, once you have a bend and a softer, flexible horse under you, who understands some leg cues. Stepping over a pole during the transition elevates her step just a bit off the ground, plants her hind feet to some extent. Then it is simply a case of trying, and rewarding her with softness and release when she happens to get it right for the first time, and a show of your happiness at her success if she manages to stagger onto the left lead for you at some point. Eureka! Don’t expect perfection at first, be happy with an approximation, be happy if she somehow manages to find it. “Make much of your pony”. Let her know that she has done something that makes you happy. “GOOD GIRL!”.
Step 4) Until you can develop the neural pathways to GET that left lead depart sequence of feet, a racehorse will usually get their flying changes, so you can practice cantering on the left lead by starting on the right lead, heading across the diagonal of the arena, and asking for a flying change onto the left lead. Because she DOES have a left lead, and can use it, she just can’t break onto it right now. So this is how you get some cantering done to the left until you have solved her transition problems. This is not a “dressage” flying change, it’s a “racehorse” flying change… you change the bend and give her a verbal “chirp” (which adds a bit of energy to her step, and she understands this). Understand that the left canter depart starts with the right hind leg, then the left hind leg and right front leg together, and ends with the left front leg (the “leading” leg). This is why you must have control of her hind end with your leg pressure. To do that, she has to understand leg pressure, which she currently doesn’t.
Step 5) Know that she is “trying”. That picking up her right lead is like you picking up a pencil (presuming you are right handed), by automatically reaching for it with your right hand. And a Martian is telling you in beeps and hisses that he wants you to use your left hand instead, and you have no idea why the Martian wants this, or even WHAT he wants from you. This is how your horse feels, confused. You are the Martian.