"Cthons are vicious humanoids that have plagued the underworld of Coruscant for centuries, though some have found their way or been smuggled offworld. Swift to mutate from generation to generation, nearly blind despite their unusual eye structure, cthons find their prey by listening for movement and feeling vibrations through Coruscant's substructures. Many of the lower level's more superstitious believe the cthons were originally Coruscanti criminals, banished from the upper levels and left to devolve. There is no scientific evidence for this belief.
While they usually scavenge from the huge piles of garbage that accumulate in Coruscant's lower levels, cthons are vicious hunters, capable of bringing down security patrols. Coruscant's reconstruction has been particularly beneficial to the cthon population, providing new lairs and breeding grounds, not to mention a surfeit of lost and easily preyed-upon refugees."
- ― Star Wars: The Old Republic, Bestiary - Cthon codex entry
Planet Coruscant Area Instance Mission