What came before Arceus?

Beginning of the Pokémon universe This information is based primarily on the mythology of both the Hoenn and Sinnoh/Hisui regions. In a vortex of complete chaos and nothingness, a single Egg comes into being, which then hatches into Arceus, the first being in existence. Arceus then creates time, space, and anitmatter.

Did Arceus created Mew?

They suggest that since Arceus is a God, they were the first. However, Arceus’ first successful attempt at creating a living creature was Mew, also making Mew, in a way, the first.