okay since mew seems to be the talk now i'll just state facts only which support why mew could be before Arceus
1. mew has the DNA of all pokemon inculdeing Arceus and the other " poke god "
2. mew is belived to be the first pokemon by many of the experts in the pokemon world becase of it's DNA which is why it can learn just about every move in the pokemon world
3. i believ one of the pokedex stated that " is the ancestor of ALL Pokemon " and unless arceus doesn't count as a pokemon , would mean he too came after mew.
no theorys please people im just stateing FACTS not things i think or feels you can check on them there stated in the games or movie so there undenyable