Slyther

Slythers were large, tentacled blob-like creatures native to Skaro.

History
During the 22nd century Dalek invasion of Earth, the Daleks used Slythers in their attacks. 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. (DW: The Dalek Invasion of Earth)

By 2199, there were still Slythers in the forests around London. (EDA: Legacy of the Daleks)

During the late 22nd century, parents that were alive during the Invasion of Earth used stories of the Slythers to scare their children into behaving. (BFA: An Earthly Child)

In the year 2515, the Daleks used Slythers during their assault on Lan Beta. (NA: Parasite)

During the 41st century, the Daleks used Slythers on Station 7. (GN: The Only Good Dalek)

Behind the Scenes

 * Terry Nation's original script called for the Slyther to be "like a huge black jelly fish" that would remain largely off-screen and never be seen in its entirety.
 * The Slyther costume was built by Shawcraft, the same company that built the Daleks. The studio's first version of the costume wasn't considered scary enough, so it was altered between episode 4 and 5.
 * The Slyther was operated by Nick Evans, who was also a Dalek operator for this story.