The Real Arceus
Beta arceus was an adorable little blob, completely devoid of comprehensible features outside of his big red oval eyes. He later became the Arceus we know in game and in movies. However, when we think about Arceus as a concept... the idea that arceus is truly the god of all pokemon starts to fall apart.
First of all - Typing.
Arceus's typing and the ability to change from one type to another was a neat concept but isn't what we expect from the god of pokemon. How does it make any sense that the literal Creator of the world, of pokemon, and of the universe requires plates to be present to change from one monotype to another? And that they cannot retrieve those plates when they're lost?
Second - the movie.
In the Arceus movie, we find out that Arceus becomes enraged at all humans because of what happens in the past. In the past we see that humans set up a trap to capture God, literal God, and somehow... arceus... doesn't know about this? doesn't foresee this? And instead they channel that anger into hating *all* humans as opposed to just... avoiding the trap? Damning the humans that tried to capture literal god?
Third - Arceus's form.
It really doesn't make sense that Arceus's form is... Like other pokemon. They're supposed to be the creator of *everything*, yet they look like a deer trapped in a fence. While you can argue that it's a form Arceus wears to be percieved by humans and pokemon, I think this is dubious at best as the other issues are still present.
So what is it then?
I believe that the Arceus we see in the movies, and in game is a puppet; a creation of the true entity meant to stand in for them while the real arceus is off tending to the Universe. Essentially, in-game arceus is an angel of the true arceus, with the purpose of presenting as God to humans and pokemon to keep their faith alive. This explains why in-game Arceus isn't omnipotent, omnipresent, nor omniscient.
So who is the real arceus, the true god? They're Beta Arceus - the true form of god - an incomprehensible mist of celestial will and power - tending to the universe, the stars, and the galaxies they create out of themselves. The real Arceus, beta Arceus, is omnipresent, omnipotent, and omniscient, but has the entire universe to tend to- so they made themselves an angel to tend to humans and pokemon alike.
We are lucky to see the face of God.