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.
The chosen one of The Sun itself
Twilight Sparkle started off as a promising magic student, shrugging off friendships for the sake of spells and history. Her teachers encouraged this, seeing the potential of ancient spellcasters within her, but it was not to last.
The Goddess of the Sun saw potential beyond sorcery, and commanded the young filly to leave school and foster interpersonal relationships with her fellow ponies.
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Through adventure and love, Twilight's magic began to manifest as powerful boons surrounding the concept of friendship itself. Rather than being the patron of the sun or moon, she had become the pillar of platonic love, creating castles and saving kingdoms with a team of ponies linked by love for one another.
The Sun believed she had reached her true potential and unlocked within her a power not seen for thousands of years. Wings and horn marked her as something beyond the three species of pony so far seen in the world. The populace called her a princess and treated her with reverence, but her friends treated her just the same.
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Ascension to Godhood is not an instantaneous process, but something she was able to process and accept as the years went on. As her power grew, so did her size, elongating into the ethereal proportions seen in the Sun and the Moon. She continued to spend time with her friends until age claimed them one by one.
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The Goddess of Friendship is the most connected to mortals of any of the deities. She treks across the globe to commune with people, give advice, and socialize, but most reliably found near her mortal home in Ponyville. She hasn't spoken aloud since the last of her friends made as a mortal passed away, but that doesn't mean she is quiet. With her growing power and size, so comes a bigger capacity for friends and relationships. While the other deities' manes dissolve into clouds, Princess Twilight's hair falls to the earth as thick ropes. These ropes are actually cables that allow a mortal pony to communicate with her, getting an immediate response and sparking conversation that can go on for hours. The fact that hundreds or thousands of ponies can do this at once does not diminish or distract from every single one of her companions and students. That is the magic of friendship. As centuries and millennia pass, her connections will spread across Equestria and eventually the world, uniting herself with ponies and people of all species, and uniting them with each other
