When everyone looked back on it after the fact, Pinkie seizing control of the chaos dimension once she was trapped in it wasn't actually that surprising. Nor was her going mad with power. Even the horn and wings made sense in hindsight.

No, it was how she won the duel with Discord that really dropped some jaws, to say nothing of what happened afterwards.

An alternate ending to IDW MLP:FiM issue #57, "Apinkalypse Now." No familiarity with the comic necessary.