"In the beginning, there was only
a churning turmoil of chaos.
At the heart of chaos, where all
things became one, appeared an Egg.
Having tumbled from the vortex, the
Egg gave rise to the Original One.
From itself, two beings the Original
One did make.
Time started to spin.
Space began to expand.
From itself again, three living things
the Original One did make.
The two beings wished, and from them,
matter came to be.
The three living things wished, and
from them, spirit came to be.
The world created, the Original One
took to unyielding sleep..."
--The Original Story, Pokemon Diamond, Pearl, and Platinum
Seven Gods and a handful of Demi-Gods maintain and shape the world in which the characters live. Arceus created the world, and populated it with most creatures, but the various Gods also had the ability to request creatures designed of their will, often choosing to request something that is representative of themselves. The results are creatures which are unnaturally strong, rare, and unable to be captured in any sort of human made capture device.
The Head Gods of the Pokemon World
These Gods are never seen by humans. They exist in their own realms and tend not to intermingle with their creations. They rule from afar, or when they do visit Earth, they are invisible to the naked eye, unable to be seen by mortals.
The All-Creator and All-DestroyerArceus - The Original One and head God of the pokemon world. He created Giratina and all the other Gods. He is also responsible for seeding the Earth with life.
Giratina - Created by Arceus to balance His power and to reign over matter, anti-matter, and the balance of things good and evil.
The Keepers of BalanceDialga - A creature created to regulate the flow of time and dictate the proper aging of new and old species.
Palkia - A creature who maintains and up-keeps various dimensions and planes of existence.
The Guardians of the ElementsKyogre - An aquatic beast who rules all the sea and has control over rain, the cold, and water. He can summon water at will and can create great tidal waves, cause floods, and manipulate still-standing water.
Groudon - A land titan capable of raising land, sinking valleys, activating volcanoes, and causing earthquakes.
Rayquaza - A magnificent air serpent responsible for the maintanance of the skies and weather. It can cause storms, make tornadoes, summon thunder, and spread clouds across the land or take them away.
Demi-Gods and their RolesThese creatures are thought to represent and preside over certain phenomenon. Although these Demi-Gods may represent these things, they generally do not have much to do with maintaining anything.
Darkrai - Considered to be a boogyman of sorts, Darkrai is said to be a fearsome demonic creature who stalks people in the night to reap their souls. Darkrai is said to be a bringer of death, as well as the being responsible for one's deepest fears and nightmares.
Cresselia - Darkrai's opposite; Cresselia is a bringer of life and good fortune. It is said to be a harbinger of luck and a giver of favorable dreams. Those close to death often claim to see Cresselia in a vision, which is taken to be a sign that their souls are not being reaped for evil but are being sent to Paradise.
Zekrom - A titan who is feared as a bringer of bad luck and despair, much as Darkrai is. It is said to represent pure, unbridled chaos, as well as pure evil. The existence of murder, thievery, and rape (behaviors in general which are considered to be sickening or wrong) are pinned on Zekrom.
Reshiram - The opposite of Zekrom; Reshiram is said to be a creature of purity and goodness. It is said to be Zekrom's foil, balancing out evil with good and helping to combat the presence of evil behaviors. It is believed to rescue people of pure souls from the brink of disasters.
Spirits of NatureThese creatures are thought to be mere legends or extinct. Human sightings are rare and rarely believed. They tend not to show themselves to humans except for orders from their masters. These pokemon were specifically created by the Gods who preside over them.
Creatures of Arceus: Victini, Virizon, Mew, Celebi, Jirachi, Uxie/Mesprit/Azelf, Meloetta
Creatures of Giratina: Mewtwo, Deoxys, Regigigas, Genesect
Creatures of Rayquaza: Zapdos, Raikou, Latios/Latias, Shaymin, Registeel, Tornadu, Thundurus
Creatures of Groudon: Moltres, Entei, Heatran, Ho-oh, Regirock, Terrakion, Landorus
Creatures of Kyogre: Articuno, Suicune, Lugia, Manaphy, Regice, Cobalion