Cybermat

Cybermats were small cyborgs used as advance guards, plague carriers and energy thieves by the Cybermen.

Physical characteristics
The various types of Cybermat seen so far have all had a metallic body, usually segmented. Some types have had eyes, some mouths, and some have had cilia. At least one type had visible organic teeth.

By some accounts, the Cybermats were created by Cybermen from bodies or body parts of humanoids not suitable for Cyber-conversion: pets, very young children and babies. (AUDIO: Spare Parts, PROSE: Illegal Alien, AUDIO: The Crystal of Cantus)

Variants
Early in the Cybermen's history, before they froze themselves on Telos, some variants had crystalline eyes and antennae through which they received commands. These Cybermats resembled oversized metallic. They had segmented bodies with cilia-like tactile sensor probes along the bases of their heads. Some, though not all, Cybermats of this kind could fit in the palm of a humanoid hand. (TV: The Tomb of the Cybermen)

In the 21st century, when the Cybermen attempted to infiltrate Space Station W3, they had photoreceptors for eyes rather than crystal-based eyes. Otherwise they looked similar to the kind seen on Telos. (TV: The Wheel in Space)

Also in the 21st century, a type of Cybermat that was buried beneath Colchester could drain electrical energy from the lights in town. This Cybermat had black, teardrop-eyes like Cybermen and an opening mouth with organic teeth. (TV: Closing Time)

When the Cybermen infiltrated Nerva Beacon in a plot to destroy Voga, the Cybermats had segmented, eyeless, flat worm-like bodies without cilia. (TV: Revenge of the Cybermen)

Another variant was a sleeker and more high-tech version of the Telosian Cybermats. Unlike the Telos version, these Cybermats were purposed with biting humans and infecting them with a virus, much like the Cybermats at the Nerva Beacon. However, instead of being deadly, the virus mutated flesh into metal and the victims became Cyberslaves. (GAME: Blood of the Cybermen)

Future Cybermen used smaller drones called Cybermites, the Eleventh Doctor described them as "Not even a Cybermat." (TV: Nightmare in Silver)

Weaknesses
Cybermats shared the Cybermen's weakness for gold. (TV: Revenge of the Cybermen) They could also be disabled with electric charges, (TV: The Tomb of the Cybermen) although this weakness seems to have been eliminated later; one was used to siphon electricity from power sources instead. The same model proved vulnerable to the sonic screwdriver, which was able to either stun or fry the Cybermat memory. (TV: Closing Time) They could also be disabled with a sonic pulse, using an amplifier. (GAME: Blood of the Cybermen)

History
The Cybermats originated on Mondas during the earliest phases of Cyberman history, when most of the planet had not yet been converted. They were kept as pets. (AUDIO: Spare Parts)

The Cyber-tombs on Telos had tunnels for the Cybermats to move from the tombs to the main control rooms. (TV: The Tomb of the Cybermen)

On GSO Arctic Drilling Station in 2010, most of the crew had been converted into Cyberslaves by the Cybermat's nanovirus. The Eleventh Doctor repelled the Cybermats with a specific frequency on a sonic amplifier device and found a cure for the nanovirus to treat Chisholm. (GAME: Blood of the Cybermen)

Several centuries before 2011, a group of Cybermen crashed on Earth. A Cybermat was used to drain electrical energy for the crashed Cybership so they could begin full Cyber Conversions. One was named Bitey by the Eleventh Doctor for its organic mouth; the Doctor reprogrammed it to drain power from the Cybership, but a Cyberman stepped on Bitey, destroying it. (TV: Closing Time)
 * Both Bitey and the GSO Cybermats were displaced in time and buried underground for centuries. Their exact chronology in Cyber-history is uncertain.

The Cybermats were used to guard the Cyberfactory when the Ninth Cyber Legion invaded London in 2012. The Cybermats were destroyed with the Cyberfactory. (GAME: The Eternity Clock)

Cybermen from the 30th century who traveled back to 1940 also employed Cybermats and harvested humans to make more of them. (PROSE: Illegal Alien)

The Cybermats appeared to have something to do with the conversion process. (COMIC: Dreadnought)

In 2503, a Cybermat smuggled itself aboard the Vanguard and began sabotaging its systems. Vol shot it, disabling its weapons, but the Cybermen reprogrammed it to take control of the crew's minds. (AUDIO: Sword of Orion)

once forced the Graak to steal a disabled Cybermat. (GAME: Destiny of the Doctors)

Behind the scenes

 * inspired the creation and look of the Cybermats. In their first appearances in The Tomb of the Cybermen and The Wheel in Space, they were realised in the studio by a variety of techniques, including clockwork and radio-controlled models. In some shots they were pulled by wires or simply pushed into view. Shots of the Cybermats leaping onto their victims were achieved by pulling the Cybermat away with wires and playing the film backwards.
 * Cybermat effects in Revenge of the Cybermen used a combination of strings, puppetry and CSO which did not look very convincing. The Cybermats (or rather the single Cybermat which appeared) went through a radical re-design and resembled a cybernetic.
 * According to The Brilliant Book 2011, a non-canonical REF book, on 30 November 1879 at Blenheim Palace, the Fourth Doctor was hunting Cybermats while posing as a Punch and Judy man for Winston Churchill.