Arceus' Circle of Creation:
Wanted to quickly touch on this and give some of my theories on the reasoning of the placement to other gods.
Before I begin, I do follow the belief that Groudon's "Supporting Gods" are Heatran, Cresselia, & Garchomp while the codenamed Mythicals are Manahy & Shaymin.
Starting in the center we of course have Arceus, the "God of All".
Directly connected to Arceus are the Universe Gods that split off from it: Dialga & Palkia (and a circle for Giratina is added on the finalized version of the circle). Dialga caused time to flow, Palkia caused space to expand, and Giratina rebelled causing it to be banished to the Distortion World. The universe is created.
On the second inner circle (here mostly covered up by the triangle formed by the Sinnoh Legendaries) are the World Gods which formed the Pokemon World: Rayquaza, Kyogre, and Groudon.
With the Pokemon World formed, Mesprit, Azelf, and Uxie are created to grant the world's inhabitant their gifts. And it is also here where we get our first major placement moment:
- Rayquaza (Sky) is placed between Uxie (Knowledge) and Azelf (Willpower): I think the reason here more lies with Lake Guardian Rayquaza isn't connected with, Mesprit. Rayqauza, as the de facto master of the Super Ancient Pokemon, is above the emotional rivalry that Kyogre and Groudon has with one another, it having the power and knowledge to quell them for the betterment of the world and the life on it.
- Kygore (Sea/Ocean) is placed between Uxie (Knowledge) and Mespirt (Emotion): As its one of the fighting Super Ancient Pokemon, it's connection to Mesprit represent its more emotional behavior. It's connection to Uxie is from its element, water, which in the Pokemon World (much like on Earth) harbors life and thus consciousness. And while this would also technically give it a connection to willpower, the seas and oceans (and currents and storms that are created from it) are a chaotic force that life as much bends to as much as it adapts.
- Groudon (Land) is placed between Azelf (Willpower) and Mesprit (Emotion): Like with its rival Kyogre, its emotional behavior is why its connected to Mesprit. As the god of the land, its connection to Azelf is one of pure might & mass. Sure, the surface of the Pokemon World is covered by 70% water, but even at its deepest the ocean only barely scratches the Pokemon World's crust as the mantle and further inner layers are made of rock, metal, and magma. And for life, land has provided stability and resources to build upon, though also provide reminders what's truly in charge with great acts of might such as earthquakes and volcanic eruptions. But while the land provides the resources, it takes life to utilize it and build something from it, hence Groudon's lack of connection with knowledge.
That leads us to the "Supporting Gods" who formed from the World Gods, each having three.
- From Rayquaza came Dragonite, Gyarados, & Tyranitar: Definitely some strange picks here, especially since it's the only one with a member who is neither a Legendary nor Pseudo Legendary (and even odder as Salamence is the only Pseudo Legendary who was excluded, though looks like they wanted Rayquaza to only have Gen I & II "Supporting Gods"). Dragonite makes the most sense here, it is a Dragon- and Flying-type like Rayquaza with plenty of legends under its belt (such as the Dragonite that flew around the globe in under a day). Gyarados at first seems like an odd pick, but if you look past the Water-type and non-Legendary (though still pretty high) stats it is a Flying-type serpent just like Rayquaza. Also, it being a Water-type links it to Kyogre, and as stated Rayquaza is the master of the Super Ancient Pokemon so makes sense to have that representation. So with that in mind, Tyranitar which made even less sense now does make some sense, it's a Rock-type theropod (dinosaurs often related to the Dragon-type in Pokemon) who symbolically represents Groudon. Rayquaza's supporters being from Kanto & Johto could one again be because Rayquaza is the master and those were the first two regions which are directly connected with one another (and while this may sound like a meta excuse, also consider a lot of in-world popular Pokemon and famous trainers are from Kanto & Johto). Infact this could be another reason why Gyarados is here, while Dragonite more represents Kanto and Tyranitar for Johto, Gyarados has a strong representation in both Regions.
- From Kyogre came Latios, Metagross, & Latias: As much as Groudon also calls Hoenn home, it's without a second guess the most water-focused region is Hoenn (until Alola, but even then...). Latios & Latias are both here as, in addition to being Hoenn's only other "additional" Legendary Pokemon, they do have a loose connection to water (their Signature item is the Soul Dew, Latios Signature Move is Mist Ball (not to mention all the Water-type Moves they both can learn), their "jet" basis can mean "jetboat" as much as "jet plane', etc.). Both of them being Dragons could also be a representation of Rayquaza. Between them is Metagross, and while its appearance and dex entries give it a robotic vibe, it's body shape is comparable to arthropods, one of the earliest forms of life that quickly evolved & widely diversified during the Cambrian explosion and even today they still populate all biomes of the seas & oceans the globe over. Metagross being a Steel-type could also be a representation of Groudon.
- From Groudon came Heatran, Cresselia, & Garchomp: While not its home turf, Sinnoh's turf has a notable landform in Mt. Coronet which makes it suitable for Groudon to represent it. Heatran's presence here is obvious, a Pokemon that is a manifestation of volcanic activities and the planet's core. Cresselia is at first odd, but it becomes more clear when you think of it representing the moon, the closest celestial body (of significant size) that orbits around the Pokemon World in a dance that has gone on for eons. Comparing it to real life counterparts, of the few celestial bodies that has parts of Earth on it it has the most, especially so when you consider the moon formed after another planet collided with Earth during the early days of the universe, Earth (obviously) surviving while the remainder of the now destroyed planet (that hasn't fused with Earth) and ejected pieces of Earth fused together to form the moon. The moon also has a connection to the seas and oceans, making it a representation of Kyogre. Garchomp is a Ground-type Pseudo Legendary, need I say more? It's also part Dragon-type which could make it a representation of Rayquaza.
Before we get to the final outer circle, there's the three "Master Gods", so called because they're direct masters of their three "Servant Gods". Each one of them is connected to a Lake Guardian:
- Uxie is connected to Ho-Oh, master of the Legendary Beasts (Suicune, Entei, & Raikou): Ho-Oh had always been presented as more "sophisticated" and "wise" when compared to Lugia who is portrayed as more savage. When Lugia's Tower burned down, Ho-Oh learned three Pokemon perished inside it and resurrected them as the Legendary Beasts. However when people showed anger and fear toward them, Ho-Oh abandoned its tower out of distrust, only appearing to humans it senses to have great potential. The Legendary Beasts serve as its heralds, doing their master's will wherever they go. It shouldn't be a surprise Suicune is the member closest to Uxie, considered the favorite of Ho-Oh and the wisest of the three beasts (hence it being the mascot of Gen II's third version). Entei leans towards Azelf's & Rayquaza's side, portrayed as the physically strongest member and the one likely to rise up to the ocassion and lead. Raikou leans toward Mesprit's & Kyogre's side, Raikou is more of a free spirit as it zips across the world with thundering rain clouds on its back.
- Mesprit is connected to Regigigas, master of the Legendary Titans (Regice, Regirock, & Registeel): It's odd to connect the God of Emotions with a group of Pokemon who are seemingly emotionless, but that's looking at things a bit too literally. Emotions can mean more than just feeling happy, sad, angry, etc., it can also mean something instinctual. Regigigas may not showcase a wide range of emotions, but what it does have is an instinct to create. It just has a natural sense of finding good materials and combining them to build working Legendary Titans (at least for the original three, when it tries to get creative and make new Titan models it seems to overreach at points). And these instincts are then also programmed into the Legendary Titans who'll follow their programming no matter what. Take Regice, the member closest to Mesprit. It's made of the physically weakest material of the three, ice, but its sheer determination keeps it from even melting in lava (maybe that's why it's seemingly Regigigas' favorite for those who played ORAS). Regirock leans toward Uxie's and Kyogre's side, it's the one that suffers the most damage due to erosion but knows how to find the proper rocks to repair itself and keep going. Registeel leans toward Azelf's and Groudon's side, with a hard steel body made from material said not found on the Pokemon World it can tank hits from either physical or special moves and push forward.
- Azelf is connected to Lugia, master of the Legendary Birds (Zapdos, Articuno, & Moltres): As I mentioned above, when compared with Ho-Oh, Lugia has been presented as the more savage of the two. By feats Lugia seems to be the physically stronger Pokemon, diving down into deep-sea trenches to sleep because an accidental flap of its wings can create a month long storm that can blow away a town. Though I suppose when you're are master of the Legendary Birds you need to have a strong disposition. Granted, while this was only in the 2nd movie (and something else had already stirred them into a rage), the Legendary Birds have just as much willpower as their master. Zapdos is the closest to Azelf just because out of the three it has the most advantages and the attitude to match. Articuno leans toward Mesprit's and Groudon's side, it goes out of its way to help people who are lost in the icy mountains that it makes its nest in (the dex is a bit obscure about this, though both the anime and manga had shown it helping). Moltres leans toward Uxie's and Rayquaza's side, it's very image having served as a symbol of inspiration and courageousness.
Finally, the most outer circle are for the "Mythicals". There are six Mythicals and they're split two for each side of the triangle that is formed by the World Gods:
- Mew & Celebi on Kyogre's side: Makes sense that Mew, the ancestor of all Pokemon, would be on the side of the element where life began. Celebi is honestly more here because they're once again pairing the Pokemon by gens. Not that time doesn't have a place on this side, if we assume the Pokemon World had similar history to Earth than as soon as water become abundant a lot of its time was spent covered in water (or ice). It also took life a long time before it was able to evolve organisms that could live on land. And while landmasses have rose up from the mantle to then get swallowed back into it, the water we have now is the same water the planet has had since water appeared (well, maybe not exactly the same, but the basic molecular structure).
- Jirachi & Deoxys on Groudon's side: Just as we did for Cresselia, we're going to have to get a bit nebulous here. Obviously the Pokemon which have connection to space and celestial objects aren't going to have a direct connection to terra ferma. Jirachi is based on making a wish on a shooting star, a shooting star being an icy comet. Deoxys is a space virus found on a rocky meteorite. But what they have similar to the Pokemon World is that they're physical solid objects made from minerals, be it a giant rock orbiting a star, a comparably smaller rock floating through space, or a clump of ice speeding by spewing its material behind it.
- Manaphy & Shaymin on Rayquaza's side: Manaphy and Shaymin both represent ecosystems. Manaphy represents marine ecosystems while Shaymin represents ecosystems with a lot of plantlife. A major aspect of these ecosystems is that they have cycles they, their resources, and the life that lives in them go through to keep thriving. And, in one way or another, these cycles at some point involve the atmosphere, usually having sent something up into the sky (via evaporation or aerial dispersal, etc.) to be filtered or taken somewhere else and then come back down to go through the process in the ecosystem before beginning again.