Cybermats
First encountered on:
Telos
Appearance:
Can vary but customarily they resemble metallic rats or silverfish.
Weaknesses:
Some have an aversion to gold. Electricity can give them 'a complete metal breakdown'.
First Appearance:The Tomb of the CybermenDoctor:Second Doctor
Last Appearance:Closing TimeDoctor:Eleventh Doctor


Overview
The Doctor has faced several versions of the Cybermats, first encountering these small, deadly robots on Telos where they assisted their masters, the Cybermen, who were attempting to escape the confines of their icy tombs. These Cybermats, like later versions, were able to move silently, leap onto their victims and proved difficult to stop. Largely impervious to bullets, the Doctor defeated them using electricity.
They later helped in the invasion of the Wheel and poisoned the crew of Nerva when Cybermen attempted to destroy Voga. The Eleventh Doctor encountered Cybermats in the Arctic and was attacked by another when he returned to England to visit his old friend, Craig.
There are several versions of Cybermats - the image above depicts the type discovered in the Arctic. Some appear to be entirely robotic but others, such as 'Bitey' have features which suggest they are cyborgs.
And then there's that silver rat thing...

Closing Time