Midnight Masquerade,
Not much is known about this pony, she only shows up once a year, at the annual Equestria Midnight Ball, She almost always wins best dress, but has never taken her mask down. Some ponies believe her to be a phantom or a sprit, but most are too shy to talk to her.
She had once even turned the head of a young Shining Armor, but when confronted, she paniced and bolted from the room.
A few believe, she is the missing filly "Bella Ball", who vanished years ago from Cloudsdale Academy, after her class mates tormented her, boasting that her mane was funny looking, and that her coat was boring and dull, mental abuse got the better of her and she flew away, tears rolling down her eyes, she never came back to the Academy again...
Story turned into myth, and at the start of the first Equestria Midnight Ball, Princess Luna was seen conversing with a young pony, a pony who went on to dazzle the guests, not only with her sliver wooves and fancy fox trot, but by the beautifull and mystious mask she wore. There was talk of her ever since, but she never stayed around long enough for anypony to say hello.
Maybe Princess Luna knows who she is, but what pony would be brave enough to ask?