Regigigas

Legends tell of the two original Pokémon, the creator of life called Arceus and the creator of the world called Regigigas. Regigigas would create and mold the land and Arceus would populate it.

This balance lasted until Regigigas created life, three helpers to assist him in forming the world for it's inhabitants. Regirock who would help form mountains and canyons, Registeel who would help form precious minerals and gemstones under the earth, and finally Regice who would help form glaciers and tundras. Arceus saw this, and grew furious. Regigigas was not supposed to create life.

Arceus, it his rage, placed a curse on Regigigas that would make him slower and weaker. Regigigas, however, was far too strong for the curse and it wore off in only 5 days when it was supposed to last for eternity. This only enraged Arceus even more. Arceus plotted to seal away Regigigas for good. He gathered energy from every species he had created and sealed Regigigas deep into the planet.

The lesser Regis tried to maintain the planet in Regigigas' place, but they could not. The fatigue of forming forests and streams and mountains and many other things made Regirock tired. This caused all rocks to turn brittle and weak against many of the elements he helped mold. Regice couldn't balance his cold regions with what life needed to survive so he froze himself a prison in a deep cavern. Registeel lost himself deep underground and simply laid limp without anyone to help guide him.

Seeing what had happened due to his sealing away Regigigas, Arceus realized he had made a mistake. But sealing Regigigas so heavily took a lot of Arceus' power away, so he couldn't simply unseal him. So instead, he formed a temple around Regigigas' sealed form, and created a life form in his shape, but made of flesh and bone and blood instead of the elements that made up Regigigas. He blessed them with great intellect, and the limbs to help them form the elements just as Regigigas had. As he retreated into the Heavens, Arceus hoped that one day one of them would be able to free Regigigas, and return him to his former power.