Rubber ponies are an emerging type of pony in ponyplay. I would classify them as a sub-type of pony under one of the above, but rubber ponies are pretty close to becoming a category of their own with sub-types of rubber ponies.
As the name implies, rubber ponies are ponies whose tack/equipment is made from rubber. Often rubber ponies will be totally encased in rubber: a rubber (often rubber latex) catsuit, rubber pony mask covering the head, rubber fist hooves, rubber bit, etc. The rubber pony combines the pony play fetish with the rubber fetish. Of course rubber ponies can also be cart, riding, show or other ponies in addition to being rubber ponies.