A BDSM activity involving the dressing up of a submissive human as a horse or pony. Frequently in pony play the submissive is outfitted with a body harness that bares the chest; a head harness that includes a bit gag, blinders, reins, and plumage; arm bondage of some sort; and foot bondage of some sort, usually involving boots that may include false hooves that hold the submissive in a digitigrade posture. Other, more sexual, accessories may include various insertable toys such as dildoes, some manner of nipple adornment, and a pony tail butt plug of some sort.
A pony girl or pony boy is the submissive in the pony play activity. Frequent activities to engage said submissives in are cart driving (Harnessed like a horse on a carriage), demonstrations of cantering skills as in a (real) pony show, and various S&M activities. Most pony play fetishists that I have heard of are more interested in the show aspects of the hobby, as it results in very beautiful presentations of the submissive. Very few dominants would attempt to ride a submissive in pony gear, as this introduces a great deal of risk of injury to the submissive. Human backs, no matter how dressed up, are not meant to bear hundreds of pounds of weight. Another unintuitive aspect of pony play is that usually the submissive is harnessed in a bipedal position, rather than being on hands and knees.
There are many pony play fetish groups online. A google search will likely turn up scores of them. It is also a popular enough fetish that BDSM porn sites will frequently do pony play photo shoots, so if you're curious about the activity, just surf around until you find a reputable BDSM porn site (They do exist!) and see if you can locate some sample pictures in their free/sample images section.
Pony play is a very expensive hobby. Far too rich for my blood, but some people have the money to burn and the drive to burn it. Expect to spend thousands of dollars outfitting a submissive in your care if you get into pony play! And take pictures! They're gorgeous!