Headshaking is a condition where the horse or pony shakes its head continuously, often while being ridden.
Originally considered a bad habit, it is now thought that some horses are allergic to certain pollens, which causes them to headshake. However horses also normally react to flies, and headshaking may be a fly defence overreaction even if flies are not present. Sometime the halter or bridle is maladjusted or irritating the animal as the rider is using rein commands.
The use of nose nets is thought to improve the problem. Nose nets are not allowed in all competitions. However British Dressage has approved of the use of some muzzle nets.