If you have ever seen a Mew movie, you know how much of an extrovert it is. And I don't know if you've noticed this, but Mew and Arceus' Pokédex entries both say they are the first Pokémon. A chicken and an egg situation. I don't want to go to into detail but, God created the chicken first.
So! Mew is floating around all by itself, really, REALLY lonely. And eventually, it came to the brink of madness. So it kind of... Well it... You know what? I'm just going to say it. It willed itself to lay an egg. And according to Acreus' Pokédex entry, it hatched from an egg and created the world. But Arceus, like any god, just had to have something to rule over. You would think it would create majorly inferior beings, right? Nooo it had to create minorly inferior beings. Like the lords of time, matter, and dreams. So smart Arceus, so smart. Maybe it's because Arceus was ignorant at that point in his life. He was kinda young. By the way, Mew, you screwed up as his mother. Just saying. Anywho he created Palkia, Dialga, and Giratina, along with the majorly inferior beings known as hu-mans. Dialga, Arceus' eldest child, became ruler of time. Palkia, became ruler of matter and space. Giratina, Palkia's twin, was banished to the dream world because of his violence and became the king of the dream dimension (Sound familiar.. *COUGH!* Hades..).

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My Theory for Pokémon Genesis
FanficOk so this is my version of the Pokémon world Genesis. If you like it, or if you don't and you just think it's plausible, throw me a comment!