After Twilight and her friends rediscover Starswirl’s last spell in the ruins of Golden Oaks Library Rainbow Dash asks, ‘why can’t we all become alicorns?’
Rarity jumped a bit, opening an eye hazily to look at what she had magicked. Somehow, a white wing was stuck underneath her. Wait. A white....wing? / Raricorn/Twicorn ship, sad later, rated for possible fluff later.
Rarity discovers that she is a very rare kind of pony; a Marshmallow pony, which are born only once every millenium. But some strange ponies appear attempting to capture her, and so Rarity must escape with all who know her secret.