Any numbers in parentheses at the ends of clues correspond to the labels of the pony anatomy in the cover image. Solving this puzzle prior to the image loading will be quite difficult. In case that image doesn't work, I put it here too: https://ibb.co/8z8DPPd
It's singular- I think they usually call it the fetlock joint. And now I see that I made the clue plural- whoops- now corrected!
A non-horse person (me) would think of it like the horse's ankle, but I guess it's not that. The horse's ankle is really the hock, and those are only on the horse's rear legs.
It seems, generally speaking, that parts on the front legs are named more in line with what we'd think of arm parts, and the rear legs are more named after leg parts; all 4 legs have a fetlock, though, which might be why they don't think of it like an ankle? Who knows. This is all Greek to me.