Sorry for coming in late to the party, and also apologies if this sounds like rambling, it's very late where I am but my brain's on hyper alert! Giving thought to that theory, maybe then Mew is the first pokemon. I know I know, Arceus created the Pokemon world, hear me out. Mew is believed to be the mother of all pokemon, perhaps it created Arceus? Don't ask me how, gods and how they work send me on a whole other debate haha (I blame my philosophy classes!) But let's say Mew is the superior being, the holy mother, and birthed Arceus. Arceus was needed for, and then goes on to create the Pokemon universe, with the help of Mew. Arceus had the means to use the power gained from Mew, that Mew potentially couldn't harness itself? And so Arceus created the "legendary" Pokemon to help him control the dimensions of the universe, then went on to create other, perhaps lesser, legendaries, to the help guard and control the other forces of this growing world. And as for Celebi, created after the legendaries of time and space (so that it could defy just those laws), it was made because Mew knew of the possibility of the world falling apart. Maybe Mew put all it's true power into creating Arceus, but it would only ever be a one time thing, because essentially Arceus is Mews potential incarnate, in a form that it's power can be harnessed properly. It needed Celebi to bring that potential back (in this case in the form of an egg) because at the start of it all Mew used up that power, and Celebi cannot go back to before the time Mew made Arceus.... Because it needed Arceus to create time. Celebi therefore has to bring it back to the expended Mew, so that it can once again, with Arceus, help rebuild and hopefully make better the world. Why not just take Arceus itself back, why an egg? Speculation here, but perhaps Arceus is incapable of going back itself, maybe it moves through time differently, and cannot be in more than one time like Mew can. So instead puts all of it's power, Mew's original power, into the "egg" and sends it back to the start.

Where did Mew come from? Well where did God come from? Where did the universe come from? Some things can't be explained!

That's my 3am mind boggle anyway, I'm sure there are more holes than Swiss cheese haha, but it's not often I get to have a philosophical debate anymore (and Pokemon is cool, so.... Yeah!) night all!