Neither unicorn nor pegasus, Alicorns appear to have traits from both species, but are so much more beyond that.
The first alicorns are known as The Two Sisters, a pair of unicorns with magic so potent, they changed the face of the sky forever. After growing wings, the young alicorns were called princesses and treated like royalty, thanked for their gift of the sun and moon. But as time went on, it became clear that these were not simply horses with extra features, but something else entirely.
When mature, alicorns are massive deities that stretch across the sky, ferrying the sun and moon to all corners of the world, leaving magic and dreams in their wake.
Being larger than the tallest mountains, goddesses generally don't interact with ponies on an individual basis. When they do, it is a terrifying honor; one that will alter the course of your life forever. Listen to the Sun speaking into your mind, and pray you are still the same pony when she is done.