An AU in which Celestia was the one to go evil and banished (to the Sun) so Luna had to take on a role of the ruler. As a result ponies eventually adapted to a nocturnal life style and went through some changes

Unicorns are now somewhat ghost inspired: their legs "melt" and their hooves are separate, covered by a foggy magical veil. Their eyes are cat-like and they have fangs
They can levitate themselves above the ground a little even without using the horn, and can also use their horn and magic overall just like the normal canon unicorns can

Twilight and Rarity as examples