whos 555555555555555555555555555555555555555555555555555555555555555555555555555555555555555555555555555555 times removed cousin
Pinkie Pie doesn't seem to have a spine either. Just look at this. [video=youtube;UDl-yZu9WlE]http://www.youtube.com/watch?v=UDl-yZu9WlE[/video]
actually we see Twilight in bandages before Derpy drops an anvil, a piano, a wagon, and a flower pot on her head. Then again, 2 minutes later she's just fine!
Personally, I think they wave cybernetics built into them, so they can stretch and twist and do very non-horse things. Actually, it is something I've given a little bit of thought to, but I suppose trying to make it realistic takes the fun out of it.