Slyther

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

Biology
Slythers looked like large, black, jellyfish-like creatures. Their skins were so thick that they were immune to the stings of the Varga plant. (GN: The Only Good Dalek) They moved slowly and regularly groaned. (DW: The Dalek Invasion of Earth)

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 last), 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 who had been alive during the invasion of Earth told 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, Station 7 contained a section of the forest on Skaro which included Varga plants and a Slyther which later returned to the Daleks. (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 Nicholas Evans, who was also a Dalek operator for this story.
 * The DVD release of The Dalek Invasion of Earth included an easter egg featuring a Slyther's "adventures".
 * The Slythers are seen by many to be the first Doctor Who 'Monster' - whilst other enemies had been mainly humanoid or robotic the Slyther was the first to attempt a completley different form of being as an adversary.