Monuments made of stone are as old as civilization. From the Pyramids of Egypt to the Great Statue of Maiden's Peak, ancient peoples around the world have memorialized their deeds and heroes in the oldest of building materials: rock. The building of Regirock was no less herculean an undertaking, but whether by accident or design, it was given life.
Regirock predates Regice and Registeel by centuries, although theories still vary wildly about the age of Regigigas. It is still unclear whether it was always made to be a weapon, or whether it was designed as a statue, but its makers stumbled upon the secret of life. Regardless, the people of Mount Coronet, often victimized by their neighbors, relied on this pokemon as their ultimate defense – the rock fallen from Arceus which all men feared to touch.
Regirock also possess another remarkable ability, quite rare among living things – one which has protected it from the wear and tear of natural monuments, which decay when their civilizations fall. They possess the capacity to rebuild themselves, and may use any stone, regardless of type or color, to do so. Yet despite Regirock's immense power, no pokemon could stand up to the conquering armies which have often trod back and forth across Sinnoh – or practically any other part of the world. Regirock was once broken and its creators were forced into exile, but from everywhere they trod, they sent back stones to aid in rebuilding their oldest protector into its current, composite form.
And then, when they again faced annihilation, they sealed Regirock away instead of sending it into battle, lest their greatest monument be lost forever.