Apparently, Arceus is in Pokémon Mystery Dungeon Explorers of Sky, albeit as a dummy.
It has the exact same Movepool as Arceus, but uses the sprite of a Substitute Doll.
I'll edit this with a video I found of it.
So, uh, discuss?
if this ends up being fake, or against the rules, though, just delete/lock this thread <_<
EDIT: Okay, found the video, apparently it's from Gamefaqs, though.
It shows up as a Psyduck on the first floor so that it's able to load properly, the same thing with it's name.
Arceus's species name is identified as a bunch of ???s, though, and Judgement's name shows up as $$$, similarly to other beta Moves.
It also has the number of 0, interestingly.
Still, it exists, which is pretty cool :P