Ya know, now that I think about it, there are more animal roles than just ponies. I've personally been trained (oh so briefly *pout*) as a cat and LOVED it. I've read (I think in the 4th Marketplace series book) of subs trained as dogs.

Why not a more inclusive tag under either the Sexual Activities or Fetish categories along the lines of:

ar = animal roles

or

at = animal training