With the way that humanity has been destroying the environment, it was decided that the time had come to start the world over anew. However the force behind the Reality Cascade was not without mercy, they didn't want to kill off 6 billion people, so the Reality Cascade was the most humane option they could come up with.
The only question was, what would be the best option to turn humans into? That decision was difficult, and it was left up to the gods of nature who drew lots in order to decide who would get the honor of ruling over the newly changed Earth and turning the Humans into their subjects.
The winner of the game was Caita, a Goddess of Horses and Unicorns. In ages past, her Unicorns had been great guardians of the forest and nature, but humans had hunted them nearly to extinction. Caita had been one of the strongest advocates for the Reality Cascade because of this, and she knew this was her opportunity to repopulate her once proud followers.
It would start small, one person would become a Unicorn, and the world around them would change. Industry would be gone, just leaving the unspoiled forest that Unicorns could call home. Even if they didn't remember being human, at least they would have their new memories and not lose their intelligence.