I'm pretty sure there are not
only Pokémon, unless "trees", "plankton" and "bacteria", in case they exist (and the trees do
, in all canons, with apparently the same variety that exists here in Earth) are all
Pokémon. There's also no reason why only humans (and trees?) have to be the only Earth-esque creatures out there when we have been shown fish in at least one continuity and there is evidence of life, Pokémon or otherwise, beyond the atmosphere of Pokéarth. Also considering something that has been said several times before — there's no way to sustain an ecosystem with such a low amount of different species.
As for Arceus, only the myth states that he created "everything", or at least that he came before "everything" (else). I'm not sure how close to the truth could that be while still interpreting the myth. He might have created only Pokémon and a Poképlanet, for example, although that gets fairly complicated. For a creature that created a friggin' universe
, Arceus was nerfed to the point of Humiliation Conga
in Movie 12 (dying because a mere meteorite hit him, or because someone dumped a bit of "silverwater" on him, duh), but then that's the anime...
edited 16th Jun '11 8:28:07 AM by SilentReverence