I think this may have been mentioned, but I will input my two cents anyways.
For all the people who says that Arceus being the creator of the universe is only a myth:
All we know of Mew being the "ancestor" of all Pokemon is its original PokeDex entry. Its PokeDex entry made by an old man who didn't even realize there was a whole other region attached to the one he was currently living in.
Anyways, to add more to the discussion, Bulbapedia has a very fascinating "history of the Pokemon universe," which basically establishes that Mew was once the only Pokemon, besides the legendaries who played a part in "creating" something (such as Groudon and Kyogre). Over time these Mews Darwinian-evolved into the other legendaries and common Pokemon we know of today, and is now elusive. So basically, "Arceus is the creator and Mew is the ancestor," as many others have said. It's really the best solution to incorporate both of their mythos. I mean, if it is to be believed, that means that Mew's original PokeDex entry is correct: Mew was the ancestor of all Pokemon...in gen 1, when they only knew about it and 150 others.