This is how Midnight becomes an Alicorn goes in The Rise of Protogirl.
[Protogirl flies through the skies above Equestria]
Protogirl: Protogirl close to accomplishing task.
Twilight will go find Midnight and destroy him.
Midnight's still alive then.
[Midnight, still in Predalien form, emerges from the ocean and undergoes a metamorphosis and becomes an Alicorn that looks like Twilight but with a dark lavender coat, a black mane with dark blue and dark purple highlights, purple eyes with turquoise backgrounds, a dark purple six-pointed star cutie mark, and dragon-like wings and flies towards land to destroy Princess Twilight]