Welcome to the O.C.I.O: where your OC can apply to enter Equestria and by your luck make it through the stringent immigration process! ...These are the ones who weren't so lucky.
Celestia wakes up as a human. But hasn't she always been that way? Or is the doctor at the mental hospital creating false memories? But...why would he do that? He just wants her to get better. Doesn't he?