Pony Play

Ponyplay is a combination of BDSM and erotic role play where the bottom assumes the role of an equid and becomes a ponygirl (or ponyboy). Ponyplay can, and often does, combine many aspects of BDSM. Ponyplay usually involves dominance and submission, and some amount of bondage and discipline. A schematic of the two main ways a human becomes a pony (this is meant to illustrative, but it is generally valid for nearly all types of ponyplay):

I would classify ponyplay into two very general types:

• "Forced" ponyplay in which the pony still considers himself to be a human but is treated as an animal and is compelled by his trainer/owner/handler to behave as an animal. This type of ponyplay typically involves humiliation since the pony is a human who is forced to take on the role of an equid and may thus feel degraded.
  Note: While it should not have to be said, it is still worth noting that in forced ponyplay (as with BDSM generally), the pony has consented to play and is not literally being forced. Ponyplay is consensual!
• "Equine role play" in contrast, is when the human "becomes" an equid and does not have a human persona while in pony role. Humiliation does not typically play a role in this type of ponyplay since animals are generally not considered to be able to be humiliated, and in any event there would be no reason a horse would feel humiliated simply for being a horse.

I don't include therians here since they truly consider themselves to be part animal (e.g. have an animal spirit). Thus, by definition it is not role-play to them. This differs from a human who enjoys non-sexualized realistic animal role play (the equine role players). Anyway, I don't want to get bogged down in details and semantics here - suffice it to say that you can think of human ponies falling into two fundamental types: those who keep their human persona, and those who lose their human persona.
The general type of ponyplay will dictate how the pony experiences the play, and also (though possibly to a lesser degree) the training methods used. In "equine role play" the methods used will very closely mirror those methods used with bio-equines. In "forced ponyplay" the training methods will often be more severe and might not be considered humane if used on bio-equines (this was not always true of course, and still isn't with same bio-equine trainers, but I would never use on my bio-equines many of the training methods used on me).
Both types of ponyplay typically involve elements of BDSM, and many ponies, even those who enter via equine role play, enjoy this aspect of ponyplay. Moreover, BDSM is natural part of ponyplay since the process of emulating a bio-equine already involves D/s, and a bio-equine's tack is, in a sense, medium to heavy bondage.
Regardless of the general type ("forced" or "equine role play"), ponies will often fall into one of three main categories:
Cart Ponies are trained to pull carts for their Mistresses. They can pull carts on either two or four legs.
Riding Ponies are trained to be ridden. Again, they can be either two or four legged ponies, and they can be ridden bare back or with a saddle.
Show Ponies are carefully trained in proper form and carriage of the body at the various gaits, or they may be used to model fancy tack and equipment.
However, many ponies will fall into multiple categories (i.e. a show pony might also be a riding pony). Again, a cart pony, for example, can be either an "equine role play" cart pony or a human "forced" to be a cart pony (i.e. a human treated like an equine and forced to pull a cart like a horse). While the end result in both cases will be a human pony pulling a cart, the pony will likely have very different experiences depending on which type of pony he is.
Also contributing to the experience of the pony is his tack or equipment. Unfortunately, ponyplay tack and equipment, just like bio-equine tack and supplies, can be very pricey. Moreover, in the last 10 years I have seen an exponential growth in the marketing of equipment geared toward ponyplay. In a sense this is good since it (theoretically) offers you a greater selection, but in reality it can be confusing if you are new to ponyplay.
While you technically do not need any equipment for ponyplay, I personally need to be wearing some tack to really enjoy a scene. Of course, for many scenes in ponyplay I have to be "forced" to remain in pony role, which obviously will require some tack for restraint and/or control. Clearly, my needs will differ from a person who becomes a pony in a different manner. However, if you (or your pony) is like me, I would suggest starting out with a bit gag and pair of standard leather wrist cuffs. Some of you may already own one or both of these, but if not, they are really not that expensive.