It is often repeated that Regigigas built Regice, Registeel, and Regirock, and the people repeating this statement mean it as proof that only the gods, not ancient peoples, could create such marvelous automatons. But this argument displays a certain lack of knowledge of Regigigas itself, for this pokemon and the civilization which built it are often considered to be one and the same.
The way that a small, isolated community with centuries of history converted itself into a single pokemon remains unknown. Some suspect Regigigas was the product of a massive human sacrifice, for its creators, defeated in war, disappeared soon after its creation; others say its massive size holds the souls of everyone who has ever lived on that mountain, or is the moving armor of its last, undying king. But the association is as old as Regigigas itself, and has been accepted by all of Mount Coronet's neighbors and conquerors. This is partially because the plant which grows on Regigigas was domesticated by the people living there and considered a local delicacy, but the knowledge of how to raise it has been lost everywhere except on Regigigas itself. Moreover, the black stripes covering this pokemon, in conjunction with its six lights, read out messages in their complex forgotten script. It is said that should any lost communities survive from that ancient nation, or (more likely in this era) any archaeologist learn to decipher their texts, Regigigas would be able to communicate as many concepts as human writing.
Regigigas retains hope that someone somewhere has learned the ancient ways which it was created to preserve, and begins every battle by "speaking" a series of messages and fighting only halfheartedly; only when it realizes its foe's lack of understanding does this forgotten automaton of a god unleash its true power.