It was hypothesized that they were the devolved descendants of the Thrint species, but the Grogs claimed to have no knowledge of their ancestors and appeared to be friendly and benevolent.
They experience two distinct life stages: the young are motile but upon adulthood, females enter the sessile life phase, and attach themselves to rocks. Remaining immobile ever after for several centuries, they enlarge, attain sentience and telepathy which helps them attracting food along with their long tongue.
Their telepathy is the most powerful in Known Space but telepathic control of an individual does not extend very far from her, and the power does not gain from being combined with that of other individuals. Grogs aren't known to use their telepathic powers to manipulate other sentients, but some humans believe they are as dangerous as Thrintun and exert telepathic control.
In appearance they are a furry cone roughly 5 feet (1.5 meters) high by 4 feet (1.2 meters) wide at the base with a bald, rounded point and a reddish coloration similar to the sands of their environment. They possess four small hairless vestigial paws, a 3-foot-wide (0.9 meter) mouth with a long prehensile tongue and no eyes or ears.
The young females being described as something like a hairy bulldog and the young male approximately the size of a chihuahua. Both possess serrated cutting edges instead of teeth. After mating, they search out a suitable location in which to settle and become sessile adults.
While Down was controlled by the Kzin Empire, and the Kzinti did not suspect their intelligence. At the end of the First Man-Kzin War, Down was liberated. In the mid-27th century, Grogs begun contact with Humans who provided them with protective equipment from ultraviolet radiation and helped them emigrate to other Human worlds.
Despite this, their threat potential was considered sufficient to demand that precautions be taken in the form of a Bussard ramjet kept in orbit around Down's sun (where it was hoped it would be out of range of the Grogs' telepathy); this would be used to create an artificial solar flare and sterilize the world of Down should the Grog take aggressive action against other species.
- Grogs at the Encyclopedia of Known Space