The Cybermats

 The Cybermats are metal rodents made by the Cybermen. The Doctor first encountered the Cybermats on the planet Telos (in the Tomb of The Cybermen). When the Cybermen plan to attack an isolated site, they sometimes send the Cybermats one ahead. Often, they go unnoticed as they slide across the floor, they seem harmless, but their true nature is soon discovered. Bill Duggan of the Wheel in Space even gives a Cybermat the name of Billy Bug before learning that the Cybermats have have corroded their way through the Wheel's supply of bernalium rods. Cybermats are capable of detecting human brainwaves, and they are known to leap onto the throat of  person, and inject them with a neurotropic virus. They are vulnerable to the broadcast of high current phase contrasts. By the time the Cybermats attack space beacon Nerva (in Revenge of The Cybermen), they are larger with a segmented carapace. This was the last time that they appeared.