well, we dont even know how alicorns... ermm... "work", as far as im concerned, they can be semidivine beings of near immortality with magical and physical skills far greater than any other kind of pony thrown into the world as agents of peace and harmony. or they could also be the last remnant of an undead caste which have kept theirselves alive with the blood of innocent griffin along the times. all this as a posture that it is too early to think of other alicorns, i mean, the princesses can be unique. there is no way to prove one way or another. until there is any official word-of-god canon answer, i say we sit back and enjoy the show, im sure it will come someday somehow (im betting Faust's DA, as has happened with other things in the series) ps: im liking my undead theory, mind you.