A well trained horse knows how to walk, trot, canter, and gallop without having a problem moving into these gaits and without fighting when asked to move faster or slower. However, this does not mean the horse is robotic and moves perfectly-without getting excited or aggravated-all the time. When asking a young, green (not fully trained) horse to move into a canter, you may experience problems; he will hollow his back, pin his ears, swish his tail, or move into a jagged, hard to post or sit to trot. This is all normal behavior for a horse that is fairly new to being ridden, Remember, like anyone learning something new, it is possible that he does not quite understand what you are asking of him and he may not want to canter with you on his back; it is a new feeling, he has to be able to balance with an extra hundred plus pounds on his back, and it is difficult to learn new things, even for a horse who wants to please his rider. These problems, no matter how annoying they may be, are all able to be fixed with more riding and practice at transitioning from trot to canter. The most important thing to remember is that you are the teacher, and just like any good teacher, you will get the most from your student if you show patience and act calmly. Instead of getting angry with your young horse, it is far more beneficial to you both, if you show your horse the same patience and support you would show a small child learning to walk. Reward small progresses and improvements, and do not over react to mistakes.
The older horse, however, may not be trained as easily. An older horse is more set in his ways and once he decides he is going to do something, it is not as easy to discourage him. A horse that has been ridden for many years and is having a problem with canter depart has a different kind of problem. He may be just acting out because he is tired of being ridden (is he being overworked?), or he may not get ridden often enough (a horse can fly off the handle easily if asked for a faster gait after not being ridden for quite some time; the pent up energy will excite him and cause him to act out, especially if he is being fed a high energy food and not working it off), has his tack been changed recently? New or different bits and saddles may be causing his discomfort. Then again, none of these may be true and he is just being difficult. First try lunging him and asking for a canter depart. Does he react favorably or does he fight? If he fights and none of his tack has been changed and he is not overworked or under worked, he may need checked over by a vet to see if there is a problem with his back. He may have pulled a muscle, or if he is a horse that is fifteen or older he may need to be retired due to a medical condition; his back may not be the reason that he is acting out; he could have arthritis setting in and he may not be able to canter without pain. If a vet check comes away clean, then he is just being difficult. You will need to ride him and school him in the arena or pasture until he stops fighting.
Each time you ask for the canter and he does not give it, or does, but bucks or misbehaves with it, reprimand him by bringing him back to the trot and working him in circles that get smaller and smaller; do this in both directions, it will not only get his mind on you and listening, it will also loosen any tight muscles in his back allowing him to move more fluently, thus giving him a smoother canter depart. A horse is an athlete, they need their muscles warmed up before they can ride well. When you get him into a canter without a fight, give him lots of praise; it will reinforce good behavior.