Headshaking is a condition where the horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

 or pony

Pony

A pony is a small horse . Depending on context, a pony may be a horse that is under an approximate or exact height at the withers, or a small horse with a specific conformation and temperament. There are many different breeds...

 shakes its head continuously, often while being ridden.

Originally considered a bad habit

Stable vices

Stable vices are stereotypies of equines, especially horses. They usually develop as a result of being confined, particularly with insufficient exercise . Vices can develop out of boredom or hunger, excess energy, isolation...

, it is now thought that some horses are allergic

Allergy

An Allergy is a hypersensitivity disorder of the immune system. Allergic reactions occur when a person's immune system reacts to normally harmless substances in the environment. A substance that causes a reaction is called an allergen. These reactions are acquired, predictable, and rapid...

 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

British Dressage

British Dressage is the organisation which oversees all affiliated dressage competitions and training in the United Kingdom. British Dressage is a member of the British Equestrian Federation....

has approved of the use of some muzzle nets.