Human pony play, also called ponygirl or ponyboy play depending on the sex of the participants, is a form of roleplay where one person acts like a pony or horse, while their partner takes on the role of rider, trainer or caretaker. The partner is likely to be a sexual partner but might not be. The pony is often two-legged: the emphasis is on the treatment of the pony, what the pony does and the mind space it is in, rather than making the person physically look like equine.
The classification of equine, which is arbitrary anyway, is rarely a distinction and 'pony' seems to be what most women think of in their fantasies. Sometimes the breed of the pony is important to the participants, especially those interested in closely mimicking behaviour. The pony is usually a woman, with a male or female partner depending on sexual orientation. Where the equine is ridden, it is more likely to be male and talked of as being a horse.
The pony may be trained to trot and walk well (i.e. in a way redolent of a true equine), and even brought to canter or gallop. They may be lunged, jumped or made to pull a cart, in which the trainer will sit, and to obey signals transmitted by the reins. The pony might have a signal of some kind to act as a safeword but would not normally speak, being limited to whinnying: this doesn't make them defenceless, however, as ponies have a natural tendency to lash out and kick when annoyed or frightened!
In a less common variant of pony play, the partner will act like a stallion. This variant may be considered an animal transformation fantasy. In this variant, the pony is often four-legged and there is a greater overall emphasis on appearance, as with dog play.
The limited evidence that is available suggests that the fantasy and practice may be somewhat more prevalent in the U.K. than the U.S.A., which may be a reflection of the number of people familiar and used to ponies and horses.
An endless array of bit gags, bridles, saddles, tack (harnesses), whips, butt plugs with tails, "hooves" that are worn like boots (sometimes also as gloves), and related items are produced and sold commercially to those who wish to act out such fantasies.
Pony play is sometimes deliberately combined or formed as part of a bondage or SM scene. This is an easy step to make since the bit acts as a gag (and may even be one) and a harness is often on the pony, restricting or controlling movement.