It is unknown who the Mysterious Alicorn really is, but her silhouette looks very similar to the figure of the Fausticorn, the pony version of Lauren Faust, creator of My Little Pony: Friendship is Magic.
Mysterious Alicorn's room is on the top of the second tower of Canterlot Castle from the right.
There's a floating painting in the secret area, once inside it, Fausticorn appears. In this room there's a tribute to the Mane 6. You can also find Smartypants.