The entirety of human civilization has flourished in a single interglacial period, yet even in this short period (at least on a geological timescale) climate has never been static. Today, snow is found only at the top of Mount Coronet; in ancient times, this mountain froze over every winter, and the caves which connected it to the outside world were frequently sealed off. It was this barrier as much as any other factor which drove monks and refugees to first congregate there. And when Regirock was beaten and the people who built it scattered, the survivors began work on a new automaton, one made from a material even more common on their mountain than rock.
If the thick shield of ice which protected and sealed away their community in winter could bar the entrance in summer from invading armies, yet step aside for trade and travel, the people of Mount Coronet thought they would never again have anything to fear. For six warm months, Coronet enjoyed peace. Yet their foes had come to see ice itself as a pokemon because of Regice; when the army came, it did in winter, applying tactics used in pokemon battles to winter ice. Before it could enter its first battle, it had already become apparent that Regice would never be the barrier that Regirock had been. Its creators even began to contemplate suicide, for their work of years had given the enemy the idea which spelled their people's doom.
And then one of them commanded Regice to strike back. Regice would never be the wall that Regirock was, but it was a far greater attacker. In an instant, a deathly chill covered the entirety of Mount Coronet, and an invading army and a good many defenders would be preserved for generations in ice.