Slythers were large, tentacled blob-like creatures native to Skaro. During the 22nd century Dalek invasion of Earth, the Black Dalek used a Slyther to guard the Bedfordshire mines at night, where it killed and ate anything it found. After it attacked a small group of humans consisting of Ian Chesterton, Larry Madison, Wells, and Ashton (killing the latter), Ian hit it with a rock, causing it to fall down a pit to its death.
The Daleks used Slythers during their assault on Lan Beta. Parents alive during the Dalek invasion of Earth used stories of Slythers to scare their children into behaving over 30 years after the events of the invasion was stopped.