Ponygirl, also spelled pony girl and pony-girl: a sexual fetish.

A ponygirl is a woman dressed in an outfit commonly made of leather, rubber or vinyl: the outfit has some relationship to equestrian tack, and generally shows a desire to transform the wearer into (you already guessed that), a pony.
Commonly, a ponygirl is made to perform certain activities, but the common feature is that ponygirl is a submissive rôle.

There are also ponyboys, although it appears to be a less common fetish.

Ponygirl gear

Ponygirl clothing and bondage items are many, but the most common are these:

• gag: usually of the "bar" type, it represents a horse bit. Usually part of a larger head harness, at times connected to reins.
• hood: occasionally decorated with horse ears. May have a hole through which the ponygirl's hair is threaded, to make a mane.
• harness: it may include arm restraints
• shoes: ranging from high heels to hoof-shaped special shoes.

More can be added (as in most BDSM things), like nipple clamps and chastity belts. One classic item is the horse tail buttplug.

Ponygirl activities

Typical ponygirl scenes and games are translations of horse-related activities like dressage and other forms of training.
The ponygirl may be made to pull a small cart or to prance in some sort of outdoor game.
Other, and harder, activities include branding and piercing.

Ponygirl fiction abounds, and it is usually not very realistic. What we are dealing with is a human being made to represent a horse, not a horse: this poses definite anatomical and psychological limits.
For example, a saddle on a ponygirl would be mostly symbolic ... on a muscular ponyboy things would be certainly different.