C is for... Cybermats

A metallic life-form employed as a weapon by the Cybermen. Their origins are unknown. They may have been manufactured, or they may be converted from some form of animal life. They have segmented bodies, tapering to a tail, and move by gliding across the floor.
The Doctor first encountered them when he joined an archaeological expedition studying the tombs of the Cybermen on the planet Telos. His companion Victoria found a small specimin in the weapons testing room, and placed it in her bag. It appeared to be dormant, but it later came to life and attacked Kaftan, who was holding Victoria captive at the time. Victoria shot and destroyed the creature.
Later, the Cyber Controller ordered a number of Cybermats be sent up a chute to the upper control room where the Doctor and the humans were gathered. They first homed in on the brain waves of the captive Toberman, presumably like a dog being given a scent. These were much larger examples. They had eyes, with a pair of antennae above.
Their method of attack was never specified. The Doctor rigged up a heavy duty power cable, which caused the Cybermats to go out of control and burn out. After the tombs had been sealed, at least one Cybermat remained at large on the surface of the planet.

Cybermats of a similar design were then used to infiltrate the space station known as The Wheel. These had a different eye-design and were without antennae. They travelled through space from the rocket Silver Carrier and burned their way into the station through its hull - their entry being registered as small pressure drops. The holes were then sealed behind them. They were programmed to destroy the station's supply of bernalium rods, needed to power the Wheel's defences. They attacked and killed one of the crewmen using some kind of sonic assault on the victim's brain. One of the creatures was disabled after being coated with a quick drying plastic. A sonic blast destroyed the others.

When the Doctor, Sarah and Harry arrived on Nerva Beacon, in orbit around the remains of the planet Voga, they found that the crew had been struck down by a virulent space plague. The Doctor discovered that this was an artificial virus manufactured by the Cybermen and spread by a Cybermat. This was of a new design, with a much longer serpentine body, and it acted more like a remote control machine. It injected the virus through a bite to the throat. Gold dust incapacitated it, and the Doctor later reprogrammed it to attack the Cybermen, having first filled it with gold dust.

The Cybermats encountered by the Doctor in a department store in Colchester were of another design. Similar in size and shape to the earliest version, these ones were definitely converted from some animal life-form, as they had savage fangs. They were used to harvest electrical energy for the crashed Cybership buried deep beneath the town. The Doctor captured one and reprogrammed it to attack the Cybermen, but it was destroyed when one of the Cybermen crushed it underfoot.

Appearances: Tomb of the Cybermen (1967), The Wheel In Space (1968), Revenge of the Cybermen (1975), Closing Time (2011).