Part I: The Birth of Ancients (18,000 - 2,000 BC)
The Arrival of Arceus
What Arceus didn't know, until recently, was that he was an alien.
Twenty thousand years ago, he plunged to the earth, housed within a massive meteorite. It was an amalgam of innumerable small plates, interlocked into a multilayered mesh that protected the soft inner core. Most of these plates, however, evaporated from the heat of Earth's atmospheric friction. The meteorite plummeted into western Asia, in what is now known as Kazakhstan, and the developing civilizations later named the great rock Zhamanshin - which is, by interpretation, the evil demon.
Indeed, the explosion left a terrible scar in the land and triggered a nuclear winter. That is, as its shards flew into the sky, the thick dust blocked the sun's light, and left a great portion of Earth to wither in cold and darkness, dooming many species to extinction in the process.
All was silent for many thousands of years. Slowly, the haze dissipated and Earth felt her mother star's heat yet again. The meteorite - now a small fraction of its original size - breathed slowly, housing a quiet egg.
The sleeping embryo was not strictly a god, but he would, in time, become like one.
While he slumbered, Earth transformed. Shards of the meteorite, which had evaporated from the heat of the fall through Earth's atmosphere, flew across the earth, settling into the dust and oceans, the trees and fields. These shards held microscopic granules of Arceus's power, ripped from him and given to Earth. Over thousands of years, animals and plants absorbed these shards, and their bodies gave way to change.
The shards bonded with their DNA, destabilizing it and triggering accelerated evolution. Animals gained fractions of godly power: fish learned to control the water around them, birds could manipulate the winds, and many others could restructure minerals or generate fire. Their bodies became larger, faster, brighter, stronger. Those unfortunate creatures who had not accessed the shards quickly enough died out, and a new wave of organisms rapidly evolved and soon dominated the world. The Earth dawned upon the age of magic, the age of power. Or, to be more precise: the age of pokemon.
Arceus emerged to this metamorphosed Earth, circa 10,000 BC, in a brilliant burst of light. He was born a small ponyta-like creature with cloud-white fur, knowing instinctively that he had no family and would need to fend for himself, but not knowing just what his arrival had done to his new home.
Because his meteorite's journey had been interrupted by the Earth, his egg had only been able to absorb a fraction of the shards that it would have, had it continued its course. He could muster weak gales of wind and fire, but had little grasp of anything else. For several thousand years, governed by his instincts, he traveled the earth, searching for any sign of the plates that should have been his birthright.
Most of the shards had been shattered to dust and absorbed into the bodies of other beings. However, one by one, Arceus found some of the lingering plates that had survived the explosion. These plates, usually around two to five meters in length, were so dense with power that any living being who attempted to touch them, would be destroyed. Arceus, however, absorbed these with ease, as his body was built to house them. And so, he recovered his powers over many millennia. In total, he was able to recover eighteen of these plates by 5000 BC, but still he knew that there were others yet to be found.
Though he had recovered much power, Arceus did not instantly realize his full potential. He grew into his powers rather gradually. By battling the pokemon around him, he learned first to command the cardinal types - water, wind, earth and fire - but eventually grasped the more subtle arts of the mind, of electric currents, of spatial distortion. Within his first six thousand years of life, Arceus mastered the techniques of every known type and was the terror of the earth. No pokemon dared approach him, for the creatures of Earth regarded him as the god of the universe.
Even more powerful than his abilities, however, was the loneliness he felt. There were no others like him, no creatures with such high concentrations of power or longevity such as his. He supposed that he would live forever on the earth, eternally without friends or family.
The humans were the nearest thing Arceus had to friends. He took a particular liking to them, as they doggedly persevered to survive, even when most other creatures were vastly stronger than them. Indeed, the shards of the meteorite had given magical abilities to most other species on Earth, but most of the humans (save for a few psychic anomalies) remained largely unmagical. It seemed that their only source of survival was their remarkable intelligence, and so Arceus did what he could to help them. He protected them from natural disasters, helped them hunt, and strengthened their settlements as he saw fit. Thus, he became a benevolent god to the humans, rather than the harbinger of death that most pokemon considered him to be.
Yet as much as he loved the humans, they were unlike him. Any human only lived, at most, for one century, and then they faded into dust, as all mortals do. And so, the loneliness stabbed at Arceus, sharper every year, until his mind went numb and he nearly lost his will to continue.
The creature who first saved him from his loneliness was, in fact, Dialga.