Pokemon Paradox (Mew and Arceus)

The Pokédex order is implied to be recorded in the order Pokémon were discovered, rather than the order they came into existence in universe. Prof. Oak was supposed to have created the Pokédex in the first game, so it is perhaps a bit narcissistic that he put his last three Pokémon first in the list.

And it makes sense legendaries would be at the end, as they would be the last Pokémon he and other researchers would encounter, being far rarer (single member species in some cases) and thus encountered and catalogued later.

There has been some debate outside the game universe regarding which Pokémon should be considered the "first" Pokémon. Mew as the first Pokémon trademarked and the supposed ancestor to all Pokémon in-game. Arceus as the presumed god of the Pokémon world (or an aspect or avatar of it anyway, as PLA implies). Bulbasaur was the original first in the Pokédex. Or the more obscure pick, Rhydon, as the first Pokémon designed and pictured in the concept art, and used as the basis for a lot of Pokémon's team screen and overworld sprites and statues throughout the world.

I think it is a mistake to try and categorise Pokédex order as an objective view on the actual Pokémon universe and the individual Pokémon's role within it, rather than simply the human perspective on cataloguing the creatures that they observe in their world.

As for the truth within the game's lore, I think there was some kind of internal legendary hierarchy chart that leaked as part of the larger Game Freak leaks a few months ago. Arceus was the creator that created the universe, and then created other legendary Pokémon that subsequently created or rule over other legendaries and trios. I don't think Mew was on it, and while it may indicate Game Freak's intent, I don't think it shows the objective truth, just the Sinnoh myths' perspective on it.

I don't think Arceus is a Pokémon. I'm pretty sure it was confirmed in PLA that the Arceus you catch isn't the real Arceus.

Short version of what I think: Mew is the ancestor of all Pokémon and the first non-Legendary that came to exist, and the one species slowly evolved into the many species we know today due to adapting to many different environments. I don't think beings who control the universe like Arceus, Dialga, Palkia, Giratina, Groudon, Kyogre, etc. are actually Pokémon. They just show up as Pokémon to interact with the world because their true forms aren't something humans can understand.

Maybe Arceus is even not really the pokémon that created the world. It is only viewed so by the people of Sinnoh. People of the Alola view Necrozma as a creator (only for the islands, yes, but I think that it kind of fits into same category). Maybe people in the ancient times of the pokémon world just viewed those super powerful pokémon as deities and creators and the myths just kind of stuck into the folklore of the regions. Otherwise I think Arceus should be more recgnized across various regins, which I do not feel it is.