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 and 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 very strong and could burst through metal structures. They lived in marshy areas. (TV: Carnival of Monsters)
Drashigs were unintelligent, but social. They hunted in packs of three and lived in colonies of twenty. When hunting or in pain, they emitted 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. (TV: Carnival of Monsters)
History
When a battlethruster made an emergency landing on the Drashigs' satellite, the Drashigs killed all fifty of the crew. The crew used weapons to defend themselves, but, save for a few scraps of the reactor venticle, the Drashigs consumed the ship.
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 they killed Kalik and Orum, two Inter Minorian government officials. However, the escaped Drashigs were all killed by Vorg with the Eradicator. (TV: Carnival of Monsters)
In a different Miniscope owned by Sabalom Glitz, there was also a Drashig habitat. (PROSE: Goth Opera)
To kill the Eighth Doctor, Ryoth used a Time Scoop to bring a Drashig to the Eye of Orion. He chose it as he deemed Drashigs invulnerable. The Fifth and Eighth Doctors built a Temporal Reverse Feedback Field that sent the Drashig to Ryoth's location, where it ate him and the Time Scoop.
The Drashig was transmatted to the Death Zone where it was hoped it would kill the other "horrors there". (PROSE: The Eight Doctors)
Other references
A Drashig striking. (TV: Carnival of Monsters)
When activated, the Master's hypnosound device which created fear-based illusions, caused Jo to imagine she saw a Drashig. (TV: Frontier in Space)
Using psychometry on the sonic screwdriver, Professor Herbert Clegg "saw" images of Drashigs. (TV: Planet of the Spiders)
Iris Wildthyme encountered Drashigs in a feeding frenzy on their homeplanet. (PROSE: The Scarlet Empress)
There was a brand of cigarettes called Drashig Cigarettes. (PROSE: Mean Streets)
One of the programs on Channel 400 was "Walking with Drashigs". (PROSE: Prime Time)
At the funeral of the Doctor, the Shansheeth used the memory weave to bring back memories of the Drashig for Jo Jones. (TV: Death of the Doctor)
According to Bernice Summerfield, the mating call of a Drashig sounded like an 18th century passenger ship horn. (PROSE: All-Consuming Fire)
Behind the scenes
Drashig is an anagram of dish-rag. (DWMS Winter 1994) Katy Manning stated this was what was generally used as a placeholder for the monster when filming. (DOC: Destroy All Monsters!)
