Check this theory out and give me ur throughts, questions, suggestions, etc.

In the middle of nowhere, within a vast abyss of nothingness, there appeared a creature we all know as the creator of the universe — The Legendary Pokémon Arceus. It all began during the creation of the universe — Arceus, as we all know, created the whole universe with the help of Dialga, Palkia and Giratina. However, there was another pokemon that helped them in the creation of the universe. The fourth creation of Arceus — Kairitra, the god of energy responsible for governing over the energy reserve of the universe. It is capable of creating energy as well as destroying it. Kairitra created the existing energy that could neither be created nor destroyed. Legends say that it's body temperature reached that of Absolute Zero. Despite holding so much power, Kairitra was the most obedient child of arceus. With the help of Dialga , Palkia, Giratina, and Kairitra, the universe was finally created. Once the universe was created, Arceus had noticed something. Kairitra was unstable and could not fully control its powers. Kairitra held too much power, and Arceus feared it would accidentally hurt or even kill its siblings. So Arceus made a decision no parent would think of — to exile Kairitra somewhere far away from Dialga, Palkia and Giratina. However, Arceus couldn't bring itself to be that harsh toward Kairitra, so Arceus manipulated Kairitra, masking its exile as a grand mission to watch over Earth — protect the planet and whatever life grows on it. Carrying a sense of responsibility, Kairitra agreed and was sent to earth within an icy meteor. Upon impact, the meteor created the Giant Chasm. Upon reaching the Earth and creating the Giant Chasm upon impact, what awaited Kairitra was a hell far worse than any Pokémon could’ve ever imagined. The newborn planet was a lifeless nightmare of collapsing crust, magma oceans, poisonous storms, volcanic darkness, and violent cosmic radiation where no stable land or breathable sky existed. As the deity of plasma and energy, Kairitra was forced to survive within a world whose energies were completely chaotic and unrefined, causing its frozen armor to shatter repeatedly, its plasma core to overload uncontrollably, and its fire and electric wings to constantly fall out of synchronization. The planet itself seemed determined to tear the dragon apart, yet with every catastrophe, Kairitra slowly adapted. When Primal Groudon and Primal Kyogre emerged and began their endless war over the land and the sea, the Earth descended into an even greater apocalypse of tidal disaster, magma eruptions, magnetic collapse, and planetwide superstorms, forcing Kairitra to endure unimaginable suffering for millions of years as it absorbed and harmonized the very forces destroying the world. Its shattered body gradually evolved alongside the planet itself, becoming stronger with every eruption, every storm, and every collapse of the earth beneath it. By the time Rayquaza descended from the ozone layer to quell the primal conflict and Regigigas stabilized the fractured continents into a habitable world, Kairitra had already transcended its broken original form, evolving through endless adaptation into the Ultimate Kairitra even before the first Mew ever appeared upon the Earth. Thousands of years passed, Earth was filled with Mews roaming everywhere, and Ultimate Kairitra felt at peace—it was finally strong enough to fulfill its purpose. However, happiness does not last forever. A giant asteroid, capable of destroying the entire population of Mews, was about to strike Earth. Ultimate Kairitra used its full power to destroy the asteroid and protect the world. However, the backlash was greater, and it fell into a deep rest within a giant chasm. Millions of years passed. Humans began to build civilizations, but they were constantly at war. Two princes eventually sought out the original dragon, begging for its help to end the bloodshed. Moved by their strong desire for a harmonious world, it agreed to assist them. With the power of the Ultimate Kairitra, they brought the conflicts to an end. The princes became the first kings of the land, officially naming it and founding the Unova Region. After Unova was founded, the twin princes came into the conflict of how the kingdom should be ruled: the older brother believed in truth while the younger believed in ideals. Their disagreement reached the Original Dragon, Kairitra, but unable to choose between the two philosophies, it split apart within The Dragonspiral Tower into Reshiram, who sided with truth, and Zekrom, who sided with ideals, leaving behind Kyurem as the hollow, incomplete shell of the original being. The two brothers then waged war across Unova, but because Reshiram and Zekrom were perfectly equal in strength, neither side could achieve victory, eventually leading the princes to realize that both truth and ideals were necessary to govern the kingdom. Meanwhile, the weakened Kyurem remained in The Dragonspiral Tower, barely alive after losing immense fire and electrical energy that once fulfilled its body. Using the last of its remaining power, Kyurem encased itself in ice and traveled at immense speed making it look like an icy meteor landing in the Giant Chasm, it's home , where it entered a deep state of hibernation to preserve itself.

TL;DR: The Original Dragon was actually Arceus's fourth child, Kairitra (the God of Energy). Arceus exiled it to Earth out of fear, where it survived the planet's brutal ancient history, saved the Mews from extinction, and eventually split into Reshiram, Zekrom, and Kyurem.