Rock Monsters are inhabitants of Planet U.
In the very early history of Planet U, the Monsters were an intelligent race, forming a civilization. It is unknown how old the civilization was, or how long it lasted, but ruins discovered suggest it was a million years old. A few thousand years ago, a enormous cataclysm reduced the Monsters to mindless brutish creatures.
After a long period, the LMS Explorer arrived in the system, and the Rock Raiders began to mine the planet's resources. They discovered the Monsters when they began exploring the middle mantle, and had many problems with the monster attacks.