Ceti Alpha Eel
A Ceti Alpha Eel is a species of parasite worm native to the planet of Ceti Alpha V. The adults appear to be predators, but the young are the greater danger, as they enter the ears of potential hosts and wrap around the cerebral cortex of the brain. Once there, they render the host very susceptible to suggestion. As the creature matures, its growth causes pain, madness, and eventually death. Once it reaches full maturity, it exits the now dead victim and presumably seeks a mate to produce the next generation.
Due to Khan keeping at least one of these creatures as a sort of a pet after the catastrophic explosion of Ceti Alpha VI, these creatures were the only known survivors of the native fauna of the planet Ceti Alpha V. Khan used infection characteristics of the juveniles as a kind of mind control to enslave and interrogate. The last victims of Khan's pet eels were Captain Terrel and Commander Pavel Chekov. As far as it is possible to tell, the last known specimen was blasted by James T. Kirk when it exited Chekov inside the Genesis test cave.