Did my best here, I'm no good with armour, but I figured that there'd be a sort of ball and socket joint on the front based on how ponies legs move and whatnot, with chain-mail for the middling joint and then heavy iron boots, along with a more heavy sort of armour for the back that's intended to keep the back legs from being easilly bent the wrong way or bashed around in general, at the cost of some mobility. We're not designing light armour here after all. And yes, it's exposed on the other side, it'd be far too hard to make a fully enclosed yet sturdy set of plates that'd hold up to that kind of stress, to my brain, anyway. And yes. it is applejack
Also, sorry this took so long, been busy.