Drashig

Drashigs were one of the most dangerous predators in the universe.

Biology
Drashigs were massive snake-like creatures with six eyes on stalks, fanged mouths, segmented bodies with greenish brown skin. They also had a sparse covering of hair. Their hide was strong enough to withstand machine gun fire, but especially large amounts of damage could kill them. They were especially strong and could burst through metal structures. They lived in marshy areas, as well as the in the caves on the 1st moon of Andrazoni. (DW: Carnival of Monsters) 

Drashigs were unintelligent, but social. They hunted in packs of three and lived in colonies of 20. When hunting or in pain, they emit a chilling scream. Drashigs could and would eat anything, including most metals. They hunted by scent and they would never give up the hunt once they found a trail. On the other hand, they would follow the trail, even when the prey had doubled back or were right by them. (DW: Carnival of Monsters)

History
When a battlethruster made an emergency landing on the Drashigs' satellite, the Drashigs not only killed every one of the fifty person crew who had modern weapons but, save for a few scraps of the reactor venticle, consumed the ship as well.

A colony of Drashigs were kept in Vorg's Miniscope. While wandering through the Miniscope, the Third Doctor and Jo Grant accidentally found themselves in the Drashig habitat. The Drashigs followed them through the Miniscope, attacking the human habitat and eventually escaping out into Inter Minor, where it was killed by Vorg with the Eradicator. (DW: Carnival of Monsters)

In a different Miniscope owned by Sabalom Glitz, there was also a Drashig habitat. (MA: Goth Opera)

In an attempt to kill the Eighth Doctor, Ryoth used a Time Scoop to bring Drashigs to the Eye of Orion. (EDA: The Eight Doctors)

Other References

 * The Master's hypnosound device, which created fear-based illusions, Jo imagined she saw a Drashig. (DW: Frontier in Space).
 * Using psychometry on the sonic screwdriver, Professor Herbert Clegg "saw" images of Drashigs. (DW: Planet of the Spiders)
 * Iris Wildthyme encountered Drashigs in a feeding frenzy on their homeplanet. (EDA: The Scarlet Empress)
 * There was a brand of cigarettes called Drashig Cigarettes. (BNA: Mean Streets)
 * One of the programs on Channel 400 was "Walking with Drashigs". (PDA: Prime Time)
 * At the funeral of the Doctor, the Shansheeth used the Memory Weave to bring back memories of the Drashig for Josephine Jones. (SJA: Death of the Doctor)