Rainbow Dash finds a maimed, unconscious young colt on the edge of Ponyville and rushes him to the hospital, but nopony knows who he is and something about him seems off...
A human soul is stuffed into King Sombra's body. Having no idea what's going on, and thinking he is in some kind of coma, he just casually strolls into the Crystal Empire.
A human was transported to Equestria and transformed into a pony. Unfortunately, he needs to get back home, and Equestria doesn't look the same as in the show.