Also, this isn't something I have thought up on by myself, yet I think it's a very interesting thought about the creation of the pokémon universe)
This topic is more concerning Arceus, Unown and the creation of the pokémon universe. As HGSS clearly showed, there's a clear link between Arceus and the Unown. But exactly how deep is this connection? Did Unown create Arceus?
Let's backtrack a bit, to the Ruins of Alph. It is said that the Unown are the cause of the Pokémon Communication Center and the signals it's transmitting. First appearing in Crystal, the text left behind by the tribe leaves us some intriguing thoughts:
Our words shall remain here for the ages
Thus we shall erect a Pokémon statue outside
They possess great insight and refuse the outside world
We humans must learn to walk in harmony with them. We depart for their sakes
(The HGSS wording is a bit different, but tells pretty much the same)
First of all, why did they have to leave? For the Unown's sake, but that's hardly describing at all. You'd think there was a special reason to this. As if that wasn't clear enough, they created a pokémon statue outside. What is that pokémon? In HGSS, Rhydon comes to mind from looking at the statues, but that doesn't explain what the statues are doing in the Sinjoh Ruins for once (in Crystal, the statue was generic). Is this some ancient pokémon? It has a link to the Unown, yet it isn't Arceus either, so what is it? Lastly, "they posses great insight". To what? They are shy, but maybe this is linked to what they already know.
As the tribe moved to the Sinjoh ruins, and there met people from Sinnoh, probably with knowledge of Sinnoh myths. That doesn't explain why the exact statues appear here and in the Ruins of Alph, considering how they made it before they left. That excludes parts of Sinnoh mythology, and it's obviously not Arceus. Yet what has Johto mythology with Sinnoh? More than is apparent, at the very least.
Turning now, to Arceus and the plates. Now, the real creation myth doesn't involve the Unown, but it doesn't exclude them either. However, the plates tell us something more:
"When the universe was created, its shards became this Plate."
"The power of defeated giants infuses this Plate."
"Two beings of time and space set free from the Original One."
"Three beings were born to bind time and space."
"Two make matter and three make spirit, shaping the world."
"The Original One breathed alone before the universe came."
"The powers of Plates are shared among Pokémon."
"The rightful bearer of a Plate draws from the Plate it holds."
The second line is the real mystery here. Defeated giants (although I think using plural here is misleading, as it is translated from Japanese.), what are they? They aren't mentioned anywhere else, and they have no connection to other pokémon, as there is no pokémon that fits this description as of now.
If you have been reading, maybe you can see some similarities. If the Unown created Arceus, they would have knowledge about the defeated giant. And they passed this knowledge to the tribe in the Ruins of Alph, who then proceeded to create the statue. Is this what caused them to leave as well? It's plausible.
What can be theorized is that this missing pokémon, this giant (a term that wasn't introduced before gen IV), was supposed to have been in pokémon Crystal, yet was dropped? However, the very idea that it is linked with Arceus was not introduced until HGSS, which is odd, yet makes some sense. Why add something when you don't build on it? I can see how the developers didn't want to create a pokémon deity as early as in Crystal, as the story that's clear in HGSS implies. Maybe that's why it was dropped in the first place?
Not to form a conclusion, because there's other parts of this chain of thought that should be added, but from what I've seen, I think too many fans accept the easy explanation without thinking about the holes in that theory. It certainly doesn't involve the Unown, and with the link between them and Arceus, that's a mistake.