There are three main types of human ponies: show ponies, cart ponies and riding ponies. My area of experience is show pony training (specifically ponygirl); in this, a ponygirl is judged on her form, her gaits, and her style. Cart ponies are trained to pull small carts, and riding ponies are meant to actually be ridden. Typically, cart ponygirls work in pairs. Ponyboys can be any of the three types, but are often cart or riding ponies; statistically less ponygirls are able to give rides. A fourth type of pony which is gaining popularity is the rubber pony. 

That's not to say a pony is a mindless, obedient slave. Ponies, both four- and two-legged, have minds of their own. Do something to startle or anger the pony, and you may be dodging a kick. 

Ponygirls seem to be more common in England than in the U.S., but there are plenty of American ponies to be found. The gear is typically a corset, boots, a body harness, a tail, and a headstall. Boots which resemble horse hooves are not uncommon. Plumes in the hair are also common. 

Pony Trainers (like yours truly) don't necessarily wear any different sort of gear. However, English riding clothes are never out of place--a hunt cap, a riding jacket, tall boots, and breeches, along with a conservative tie for men and a choker and pin for women.

 In pony training, the Trainer shouldn't be sadistic towards the pony, but should be firm, just as with a four-legged pony. A smart Trainer can and should be able to keep an uppity ponygirl under control without losing his or her cool. Simply use the reins and crop to keep things moving along. A good ponygirl will be able to deal with the pony status: ponies don't communicate except for neighing or whinnying (i.e., no talking), having her hands bound behind her back during training times, and in general being treated as an animal rather than as a person.