Cybermat

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

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 silverfish and had segmented bodies with cilia-like tactile sensor probes along the base of their heads. Some, though not all, Cybermats of this kind could easily fit in the palm of a humanoid hand. (DW: 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 crystalline based eyes. Otherwise they looked similar. (DW: 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 also had an opening mouth with organic teeth. (DW: 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. (DW: Revenge of the Cybermen)

Another variant was a sleeker and more high-tech version of the Telos version of the 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. (VG Blood of the Cybermen)

Weaknesses
Cybermats shared the Cybermen's weakness for gold. (DW: Revenge of the Cybermen) They could also be disabled with electric charges, (DW: The Tomb of the Cybermen) although this weakness seems to have been eliminated later; one was used to siphon electricity from power sources instead. (DW: Closing Time) They could also be disabled with a sonic pulse, using an amplifier. (VG: 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 converted. (BFA: Spare Parts) The Cyber-tombs on Telos had tunnels for the Cybermats to move from the tombs into the main control rooms. (DW: The Tomb of the Cybermen)

On GSO Arctic Drilling Station in 2010, most of the crew had been converted into Cyberslaves through 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. (VG: Blood of the Cybermen)

Several centuries before 2011, a group of Cybermen crashed on Earth and 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 due to its organic mouth. The Doctor reprogrammed it to drain power from the Cybership, but a Cyberman stepped on Bitey, destroying it. (DW: 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.

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

Some accounts showed the Cybermats were created by Cybermen from bodies or body parts of humanoids not suitable for Cyber-conversion: pets, very young children or babies. (BFA: Spare Parts, PDA: Illegal Alien, BFBS: The Crystal of Cantus)

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

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

Behind the scenes

 * Wood lice 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 silverfish.
 * 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.