I'm a big fan of multiverse theory, so I like to envision there being an infinite number of pokemon universes with their own Arceus. Each one is their own person with their own names and with slight variations from each other and can visit and communicate with one another at will. Some have even gone so far as to alter their physical appearance in some way. In the pokemon universe where my characters reside, its Arceus is plagued by boredom and does things on a whim. While surveying the planet, he happened upon the mega Y form of a Mewtwo. He took so much of a liking to it that he went down and fused with it, gaining the form he has now. All I got for now.