If you are like me, equine anatomy was a completely alien subject before Friendship is Magic; in fact, I always considered horses to be relatively ugly beasts. I still do! But these ponies sure as hell don't look like any horse I've ever seen!
Hopefully the above chart helps explain some of the strange terms that keep popping up, primarily in Art of the Dress where rarity pretty much goes down the list of various body parts to measure on her friends.
(Withers is way less dirty than I originally thought...)