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. Cybermats like this were almost unnoticeable. (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, spines down their backs and lacked the antennae. 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 had been 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. They had needles which injected poison into humans, but they could also inject gold dust into Cybermen. (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, which the Eleventh Doctor described as "Not even a Cybermat." (TV: Nightmare in Silver)

Weaknesses
Cybermats shared the Cybermen's weakness for gold, but it was possible for gold to be placed inside them without damaging their systems and the gold could be injected into Cybermen as they would inject other humanoids. (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) The Cybermats of the Mondasian colony ship contained a failsafe system (that they also shared with the patients and the fully converted Cybermen) that, if turned on, would force them to stay on Floor 1056 of the ship; their programming could be hacked to deadlock this system on, trapping them at the bottom of the ship. (PROSE: Alit in Underland) However, the Cybermen were eventually able to work around this and deactivated the system, allowing the Cybermats to function properly again. (PROSE: Twice Upon a Time)

The Mondasian colony ship
Cybermats were used by the Cybermen created by on the Mondasian colony ship. One group of Cybermen, under orders from a Cyber-Planner, attempted to use a swarm of Cybermats to attack the colonists of Floor 0507 so that they would be easier to capture for conversion. However, was able to use the ship’s systems to hack into the Cybermats' programming, allowing her to reactivate an old failsafe system that her previous incarnation built into the cyber-technology of the hospital on Floor 1056, causing the Cybermats to return to the bottom of the ship and remain trapped there. (PROSE: Alit in Underland) Eventually, however, this system was deactivated again by the Cybermen. After Nardole and the colonists of Floor 0507 evacuated to Floor 0502, large groups of Cybermats continually traveled to their location, arriving at the solar farm every spring for several centuries, even after Nardole's  death, though by that time the colonists had “sorted” their conflict with the Cybermen. (PROSE: Twice Upon a Time)

Mondas
Cybermats were also developed 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)

21st century
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)

In 2011, a group of Cybermen who had crashed on Earth centuries before were able to make their way to the surface. 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)

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)

23rd to 25th century
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)

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) Cybermats were also seen on the surface of Telos. (GAME: The Doctor and the Dalek)

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

Far future
When the Cybermen allies with Rassilon to change history, a swarm of Cybermats covered the Doctor's TARDIS when the Eighth Doctor stepped out. (COMIC: Supremacy of the Cybermen)

Undated events
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. 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 reference book that is not considered a valid source on this wiki, on 30 November 1879 at Blenheim Palace, the Fourth Doctor was hunting Cybermats while posing as a Punch and Judy man for Winston Churchill.
 * The s are minor villains in LEGO Dimensions.