The rock beasts were non-intelligent lifeforms native to a class M planet near Motherlode. Two varieties are known to exist: a four-legged type and a two-legged type.
The four-legged beasts had a long snout with teeth, two eyes, and four fingers. The two-legged beasts, which crawled on their tail-like back end, was short-snouted with teeth, three eyes, and had only two fingers. Both were similar in temperament, roar, and a tendency to assume an upright posture. These creatures were extremely large in comparison to humans, had tough skin that made them resemble rocky mounds when inactive, which allowed them not to register as "living" to Federation sensors.
In 2269, they menaced an away team from the USS Enterprise which was searching for Nurse Chapel, who had been kidnapped by Harcourt Mudd. (TAS: "Mudd's Passion")