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)
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 it was killed by Vorg with the Eradicator. (TV: Carnival of Monsters)
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.