It is a little-known fact that, like Porygon and Mewtwo, Golem is an artificial pokemon. The weak, especially the weak and unusual, have always needed protection from the strong, and for a religious community on Mt. Coronet which prayed to a Probopass god, the danger was all too real. Their god, who they had absolute faith in, had failed to protect them, but they would not renounce him even at the cost of their lives.
If only they could command stronger pokemon. They defended themselves with what was available in the area, but their imaginations were gripped by the story of the Regis, three ancient and powerful machines created by another priesthood, in another time – artificial pokemon which themselves were eventually worshiped as gods. Their secret was lost, but perhaps if something similar could be done with Graveler, they could find a way to fight back.
And something could be done. Through the usage of large machines traditionally used for trading pokemon and a great deal of experimentation, a method was found to turn Graveler's uneven rocks into an immensely durable series of interlocking, hexagonal stones, and its body shape became one which could fit into a cannon. Two arms were lost, but the other two gained in strength. The new pokemon was named Golem, after the phrase "legendary golems", an old term for the Regis.
The community which invented the Golem wiped themselves out in a mass suicide after a desperate last stand. Their secrets were spread far and wide throughout the occupying empire, and from there through espionage and captured machines throughout the pokemon world. In these times, the ability to evolve Graveler into Golem is a standard feature for trading machines, and its roots have been all but forgotten.