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Azelf, Mesprit and Uxie, the spirit. They are the more precise product of what the Original One wanted beings to be. And thus, they will infuse humans with their powers to influence their minds. They too, like Arceus, own a power of control through the red gems in their head and tails, their stone and core, which form the red chain; specifically designed to control and restrain the 3 dragons, as well as the caves where the 3 pixies reside, forming a triangle around the spear pillar and with their power sealing away the 3 dragons. Causing, paradoxically, their same containment as well, destined to stay slumbering in the caves forever to trap the dragons and sustain this existence of slavery, with only their spirits being able to be awake on the higher parts of the lakes.
The Unown, the cells and eyes of the Original One, which respond to Arceus's will and serve Arceus by forming new universes when they reunite to form one of the 3 dragons, and by either allowing it to look over the universe, or rather by communicating directly with Arceus.
Regigigas, the body. What seems like being the base body and shape of the Original One, or a product of a blank plate, a piece of the body of the Original One. It protects the world from destruction and disaster, and represents a Giant in miniature, as well as the other Regis created by him by wielding the elements found in the world or in the plates.
The plates. Pieces of the Original One's body that have absorbed the powers of the Giants. They're the source with which Arceus still manages to absorb the Giants' powers, probably now scattered too much to contain enough power for him to absorb.
There could be other entities or parts resulted in its division, such as Zacian and Zamazenta's anti-giant armors, Calyrex and the shards found throghout Sinnoh, which could be either smaller leftover pieces of the Original one and/or of the Giants.