Dildoes and strap ons abound both in and out of BDSM and are by no means unique to ponyplay. However, one "class" of dildoes seems particularly well suited to ponyplay: equine dildoes.
Equine dildoes are simply dildoes that are designed to be very similar anatomically to a horse's cock. A horse dildo is not simply a large (many are less than life size) insertable, it is the same shape and, in some cases, coloring of a horse cock. Most often, it is the shape of the head which is faithfully reproduced with the overall size being reduced in both length and girth (although full size versions are available, and in those cases, the medial ring is typically reproduced as well).
Incorporating these dildoes into ponyplay can add an additional degree of realism to your scenes. While the added realism afforded by these may be too much for some trainers and ponies, I take the view that these dildoes are merely toys (which they are) and can be a nice addition to some breeding scenes. Personally, I am a little fascinated (and a little scared - see photo at right) by these. I'm not sure if I'd want the full sized version used on me...well at least not all the way.
That equine strapon is not just for show. If you don't believe it's possible for a person to take the nearly 20 inches of horse cock dildo shown in the photo, watch this video of the strapon in action.
There are a wide variety of equine dildoes available, which vary in how accurate they replicate the features of an equine penis. Most will replicate the overall shape of the head fairly well and many are available in realistic colorings. The largest (excuse the pun, but I couldn't help myself) variable is the size of the dildo. Most of the ones on the market are smaller, truncated replicas (i.e. they are smaller in diameter and only include the head and the first few inches). Dildoes that are of correct girth dimensions are not too hard to find, but ones that replicate both the girth and the length are not as common (Zoo Fur used to sell the Thor, but this has since been discontinued).
The Pegasus Equine Sheath is a little different from other dildoes in that it can be worn over a male pony's own cock. It even has a hole at the head so that a pony can cum through it. This offers opportunities for some interesting and unique breeding scenes. I'm not sure about how much stimulation gets through to the wearer's own penis, but this is something I'm dying to try.
The equine sheath can also be worn by females as a strap on with additional adapters, or it can be used as a standard dildo or even a masturbation sleeve. While I have not had the opportunity to actually try it, the Pegasus equine sheath appears to offer a really good reproduction of the overall shape of a horse cock and promises to offer male ponies a unique breeding experience. The Pegasus equine sheath is on my list of things to buy, and when I do, I will post a full review. Update: I have ordered the Pegasus Equine Sheath from Fetish Zone (on 2012-08-02). You can read the complete review if you're interested.
While equine dildoes are not for everyone, I do think they can be loads of fun in some breeding scenes. Just keep in mind that the larger, more accurate reproductions are cool to look at but not necessarily the best ones for practical use (as you can see in the photo at the bottom right).
Places to buy equine dildoes: