My theory:
Before the universe was created, Mew was born. Mew being the first Pokemon, laid an egg. The egg later hatched into Arceus (Mew's offspring), in which, he thought he was the first Pokemon to be born, since Mew left to someplace...far...far away. Mew also didn't create a world for anything to live in or create something to clear the darkness, so Arceus truly believed he was the first Pokemon. Arceus later understood his powers, and created a universe, for life to live in. Eventually, Mew came back to the spot where he laid Arceus's egg, but Arceus wasn't there. Mew looked around himself, and saw a world was created, by Arceus. He saw life on Earth, a planet created by Arceus, and decided to lay more eggs, so more of its offsprings could live on Earth. Eventually he laid hundreds, or thousands of offsprings of Pokemon, and later set them on Earth. Arceus later came back, and found Mew. He surely believed this was a Pokemon he didn't create, but he didn't care either. He later created humans, and more things to live on Earth...
So basically...
-Without Arceus, there would be no universe
-Without Mew, there would be no life (same goes for Arceus)