Cthons, also known as Troglodytes, were vicious humanoids that had plagued the underworld of Coruscant

Coruscant Planet

for centuries, though some had found their way or been smuggled offworld. Swift to mutate from generation to generation, nearly blind despite their unusual eye structure, cthons hunted their prey by listening for movement and feeling vibrations through Coruscant's substructures. Many of the lower level's more superstitious believed the cthons were originally Coruscanti criminals, banished from the upper levels and left to devolve. There was no scientific evidence for this belief.
While they usually scavenged from the huge piles of garbage that accumulate in Coruscant's lower levels, cthons were vicious hunters, capable of bringing down security patrols. Coruscant's reconstruction after Sacking of Coruscant

Sacking of Coruscant Event Great Galactic War (3681BBY - 3653BBY)

was particularly beneficial to the cthon population, providing new lairs and breeding grounds, not to mention a surfeit of lost and easily preyed-upon refugees.
Cthons were beneficially susceptible to radiation, and after exposure to significant levels of radioactive isotopes, their ferocity and aggressiveness increased, their bodies began to emit a purple glow.