Cybermat

Cybermats were small cyborgs used as advance guards, plague carriers and energy thieves by the Cybermen. (TV: The Tomb of the Cybermen et al)

General
The various types of Cybermat all had a metallic body, usually segmented. Some types had eyes, some mouths, and some had cilia. At least one type had visible organic teeth. (TV: Closing Time)

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

Variants
Prior to Mondas' total cyber-conversion by Cyber-Commander Zheng, the last surviving Mondasians kept Cybermats as pets, and were simply referred to as "Mats". Some Mondasians could become "Mat Catchers". During this time, Cybermats came in numerous different versions, such as Deluxe Mark Sixes; Deluxe Mark Twelves, a black variant of Cybermat, were exclusively used by the Central Committee. (AUDIO: Spare Parts)

Before the CyberTelosians froze themselves on Telos, some Cybermats 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 of the Late CyberFaction 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)

When some CyberNomads 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) The Cybermats of the Mondasian colony ship were also capable of poisoning Mondasians. (PROSE: Alit in Underland)

One Cybermat variant - used by space-faring Cybermen resembling those of Cybus Industries - was a sleeker and more high-tech version of the Telosian Cybermats. Unlike the Telosian 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)

One Cyberman, stranded on Morabolis, used microscopic Cybermats to partially convert all administration on the planet into the Suits. These maintained control over these people, making them behave as Cybermen while resources were being gathered to fully convert the population. Since they were linked to this Cyberman, when he was killed by Sogo, all the thousands of Cybermats controlling each individual left their hosts and initiated self-destruct, restoring their autonomy. (AUDIO: Altered Status)

The Cybermen of the Cyber Legions used a type of Cybermat that drain electricity. This Cybermat had black, teardrop-eyes, like most Cybermen, and an opening mouth with organic teeth. One such Cybermat was used by the Cybermen of a crashed Cyber-ship to harvest electricity from the cabling underneath Sandersons and Granger on Earth in the early 21st century. (TV: Closing Time)

Similar counterparts
Later, during the Cyber-Wars, (PROSE: A Girl Called Doubt) the Cyber Legions used smaller drones called Cybermites, which the Eleventh Doctor described as "not even a Cybermat anymore". (TV: Nightmare in Silver) These highly-evolved Cybermats were often seeded into planets conquered by the Cyber Legions, and reproduced naturally by harvesting minerals from the planets to construct more Cybermites; this process would be repeated by the newly-constructed Cybermites, and after several months, or even years, the Cybermites would number in the billions, with the power to successfully fully convert people into Cybermen. (PROSE: A Girl Called Doubt) Cybermites could also perform more traditional Cybermat functions, such as harvesting electricity or being highly covert spies. (TV: Nightmare in Silver)

Mechanising Worms, which were used by the Cybermen of the Cyber-Mainframe, (AUDIO: Telepresence, Code Silver) were highly reminiscent of Cybermats, in particular the Cybermats used on Nerva Beacon. (TV: Revenge of the Cybermen) Unlike Cybermats, however, they could be broadcast across dimensional gaps, and could perform partial conversions as part of the creation of a dimension bridge between universes, (AUDIO: Telepresence) or for full conversions, the latter of which could occur if a dead Cyberman was salvaged; each Cyberman contained a swarm of Mechanising Worms, which would attempt to upgrade the Cyberman's attackers to create replacement Cybermen. (AUDIO: Code 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 inherited from the patients, and was shared with the evolved Cybermen - that, if turned on, would force them to stay on Floor 1056 of the colony 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
The Cybermen of Operation Exodus, who evolved from the Mondasians of Floor 1056 onboard a Mondasian colony ship, (TV: World Enough and Time, The Doctor Falls) developed Cybermats. A group of CyberNeomorphs, under orders from a Cyber-Planner, travelled to Floor 0508 and attempted to use a swarm of Cybermats to poison the Mondasians of Floor 0507 so that they would be easier to capture for cyber-conversion. However, was able to use the colony ship's systems to hack into the Cybermats' programming, allowing her to reactivate and deadlock an old failsafe system that  built into the patients developed in 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, (PROSE: Twice Upon a Time) as after Nardole and most of the Mondasian evacuees of Floor 0507 evacuated to Floor 0502 following the Battle of Floor 0507, (TV: The Doctor Falls) large groups of Cybermats continually travelled to their location, arriving at the solar farm every spring for several centuries, even after Nardole's death, though by that time the Mondasians had "sorted" their conflict with the Cybermen. (PROSE: Twice Upon a Time)

Mondas
Cybermats were also developed on Mondas before the last surviving underground Mondasian city been converted. They were kept as pets. The Fifth Doctor reprogrammed a swarm of Cybermats to feed off their electricity supply of the Central Committee of Mondas, killing them. His efforts were, ultimately, in vain as eventually all of the Mondasians underwent forced cyber-conversion. (AUDIO: Spare Parts)

Late 19th and early 20th centuries
In 1879, on Winston Churchill's fifth birthday, someone that young Winston interpreted to be a Punch and Judy man arrived and began tracking a miniature crocodile. The man, who wore a long multi-coloured scarf, called Churchill "Winnie". Churchill found himself enjoying the company of the strange man. The figure, actually the Fourth Doctor, eventually caught and smashed the crocodile, actually a Cybermat, before disappearing in his Punch and Judy box. (PROSE: The Lost Diaries of Winston Spencer Churchill)

In the mid-20th century, the Cybermen of Mondas used caterpillar-like creatures which Ace identified as drones. These drones were fitted with fire extinguishers and were capable of loading cargo. (COMIC: The Good Soldier)

Cybermats appeared on a planet which the CyberMondasians had occupied. The Second Doctor and his grandchildren, John and Gillian Who, found three dead Cybermats in a meadow of flowers. Determining that the Cybermats had been killed by the unique scent of the flowers, the three travellers used the flowers to get past the Cybermen and escape their Cyber-City. (COMIC: Flower Power)

Late 20th century
Following the first invasion of Earth, Gareth Arnold uncovered a Cyber-ship that had crashed in Cambridge many years beforehand and experimented with the technology that had survived. Using Cybermats, Arnold attempted to convert all of Cambridge, but was stopped by the Third Doctor. The Cybermen that Arnold had created went into stasis on the ship, awaiting an activation signal. (AUDIO: The Blue Tooth)

One Cyberman managed to cannibalise garbage it found in the London sewers to repair itself and create a considerable amount of Cybermats. The Third Doctor and Jo Grant stopped it from infecting an entire hospital with Cybermites. (PROSE: The Piper)

Early 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)

Late 21st century
During the late CyberFaction's attack on Space Station W3 in the year 2079, Cybermats were deployed by the Cybermen following the orders of the Cyber-Planner (The Wheel in Space). These Cybermats proceeded to consume the space station's store of bernalium need to power the x-ray laser, leaving the station vulnerable to meteorites. They were discovered by engineer Bill Duggan before scurrying away. Elsewhere, the Cybermats attacked and killed engineer Kemel Rudkin. Ultimately, the Second Doctor neutralised the Cybermats by jamming their audio frequency, causing them to disintegrate. (TV: The Wheel in Space)

23rd century
The Cybermats appeared to have something to do with the conversion process. (COMIC: Dreadnought)

Telos
The Cyber-tombs on Telos had tunnels for the Telosian Cybermats to move from the tombs to the main control rooms. Along with the CyberTelosians, these Cybermats lay dormant until Telos was found by the Brotherhood of Logicians. A small Cybermat was found by Victoria Waterfield. When the Cybermen were reawoken, this Cybermat became active and attacked the Kaftan as she held Victoria at gunpoint, rendering the Logician unconscious and inadvertently saving Victoria, who destroyed it with Kaftan's gun. Larger Cybermats were used by the CyberTelosian Controller to attack the humans after he and the other Cybermen were trapped underground by the Second Doctor. The doctor was able to disable the Cybermats with an electric cable. (TV: The Tomb of the Cybermen)

Cybermats were also seen on the surface of Telos when the Twelfth Doctor sent "Lumpy" the Dalek there to retrieve one of the pieces of the Orb of Fates, which was hidden in some ice.. (GAME: The Doctor and the Dalek)

Nerva Beacon
Cybermats were used by the CyberNomads in their attack on Nerva Beacon. Transmitting venom with a bite, these Cybermats first infected crewman Warner, who soon died. A Cybermat then attacked Sarah Jane Smith, companion of the Fourth Doctor who killed the Cybermat with gold dust before curing Sarah. Taking control of a Cybermat, the Doctor used it to threaten Kellman, the Cybermen's human ally. Later, the Doctor filled the Cybermat with gold dust and used it to a Cyberman, injecting it with the dust and killing it. (TV: Revenge of the Cybermen)

Orion War
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)

After the Cybermen took control of Earth, Cybermats were deployed in the sewers to find any members of the underground resistance. (AUDIO: Outsiders)

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)

54th century
During the 5320s, the Cyber-Empire had a Cybermat delivered to Addison Delamar so they could harvest the Ninth Doctor's memories of the Last Great Time War. The Cybermat managed to partially convert Delamar before Tara Mishra used gold to destroy it. (COMIC: The Bidding War)

Third Cyberwar
During the early years of the Third Cyberwar, Cyberiad-type Cybermen sent hundreds of Cyber Legion Cybermats to attack Space Station Hamlyn. However, the Cybermats were all lured to defeat by the Second Doctor, using his recorder. (COMIC: The Scruffy Piper)

Undated events
On the planet Yama-10, the Cyberman planted hundreds of Cybermats capable of squirting poison on a camp of Ice Warriors led by Commander Yinak, who ordered their destruction. Though the Cybermats were believed to have all been destroyed, the Ice Warriors soon came into conflict with the Cybermen themselves. (COMIC: Deathworld)

A Cybermat attended Bonjaxx's birthday party at Bonjaxx's Bar on Maruthea, which resulted in a bar fight. (COMIC: Party Animals)

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

once goaded the Twelfth Doctor into following her through various time periods while she stole valuable items. The Doctor was too late to stop her each time. Finally catching up with her in the Stone Age, the Doctor revealed to her that he had discovered her true plan: to leave Cybermats behind where she stole each item. Missy expressed her desire to further reveal her plan to him. The Doctor instead refused to listen and boarded the TARDIS in search of some lunch. (PROSE: Dr. Twelfth)

Alternate timeline
In an alternate timeline where Rassilon and the Cybermen conquered all of time and space, the Eighth Doctor and Josie Day encountered Cybermats, along with some Cybermen, on a spaceship. In particular, a swarm of Cybermats covered the Doctor's TARDIS when the duo attempted to escape. (COMIC: Prologue: The Eighth Doctor) This event was erased from history following the Twelfth Doctor and Rassilon’s efforts to regenerate the universe through the Eye of Harmony. (COMIC: Supremacy of the Cybermen)

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.
 * Cybermats identical to Bitey appear as enemies in the video game Doctor Who: Legacy. They feature in both closed and open mouth variants, with the latter being enlarged for an additional giant variant.
 * The Cybermats appear as minor villains in LEGO Dimensions, sporting a design that has not yet been used in any other Doctor Who-related material.