How do you fix a horse that drifts laterally instead of traveling straight?

A horse that drifts — consistently traveling at an angle rather than in a straight line, or whose hindquarters trail to one side — is a horse with either a physical asymmetry or a training hole in its straightness, and identifying which is driving the drift is the correct first step. Physical causes of lateral drift include: one-sided muscular development from asymmetrical training or work demands, chiropractic issues in the spine or pelvis that make traveling straight uncomfortable, vision problems where the horse is tracking off its better eye, and soreness in one hind leg that causes the horse to carry its hindquarters to the less painful side. Anderson recommends a veterinary evaluation for persistent drift, especially if the drift developed suddenly in a horse that was previously straight. For training-based drift, Anderson's correction uses the leg aids on the side toward which the hindquarters are drifting. If the hindquarters drift right, the rider's right leg behind the cinch asks the hindquarters to move left and straighten. This is the same lateral movement used in haunches-in but applied as a straightening correction rather than as a lateral exercise. He also teaches riding toward specific targets — a fence post, a letter, a cone — rather than riding in a general direction, because riding toward a fixed target gives both horse and rider a reference point for straightness that is otherwise absent. A horse that drifts when ridden in open space often improves immediately when given a visual target to track toward. Parelli adds the shoulder-control component: many horses that drift in the hindquarters are actually drifting their shoulders first, and the hindquarters follow. Checking whether the drift begins at the shoulder and correcting there — using the direct rein to bring the shoulder back into the line of travel — often corrects what appears to be a hindquarter drift problem.