Yernoids are man-sized bipedal reptilian creatures (smaller versions of the Eorna) which have developed a rudimentary intelligence which include tool and weapon making. These creatures usually roam in the caves and underground of the badlands. They are sensitive to light. They are territorial; the strongest Yernoid always leads the group.
In truth, the Yernoids are devolved Eorna that have been living in the underground tunnels since the war ended. Between the radiation from the Sathar bombs and the corrupted energies of crystals, these creatures mutated out of a population of Eorna that hid in the underground shelters. Unfortunately, they died off and all that was left was their mutant offspring.
They now have a vague memory of what happened and tell the tail on their walls with their paintings. In essence, they tell of a great war with the evil worms, the devastation of their weapons, and their attempt to hide in the catacombs of this facility. They tell of their great forefathers/mothers and how big they were. They gave birth to the Yernoids and then died out.
Since then, the Yernoids have been living in the catacombs, only occasionally being pestered by local fauna and the Zethra who likes to enslave them mentally.