Slyther

Slythers were creatures native to Skaro. They were occasionally utilised by the Daleks to act as guards at their installations.

Biology
Slythers looked like large, black creatures. Their skins were so thick that they were immune to the stings of the Varga plant so they wouldn't transform into a Varga like other lifeforms. (COMIC: The Only Good Dalek) They moved slowly and regularly groaned. (TV: The Dalek Invasion of Earth)

History
During the Dalek occupation of Earth in the 22nd century, the Daleks used Slythers in their attacks. (AUDIO: Masters of Earth)

Towards the end of the invasion, the Supreme Controller 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. (TV: The Dalek Invasion of Earth)

By 2199, there were still Slythers in the forests around London where they would attack anyone who wandered into their paths. Dedicated knights hunted and killed them. (PROSE: Legacy of the Daleks)

In the year 2254, the Seventh Doctor encountered Slythers on Skaro. (GAME: Dalek Attack)

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. (AUDIO: An Earthly Child)

During the Great War, a Slyther was used by the Daleks on the planet Antalin. (PROSE: War of the Daleks)

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

While on Skaro in the 27th century, Bernice Summerfield commented that she would rather hug a Slyther than have her nerve fried by a Dalek gunstick as it would be quicker and less painful. (AUDIO: The Lights of Skaro)

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. (COMIC: The Only Good Dalek)

Following the reconstruction of Skaro in an era that succeeded the Last Great Time War, adopted a Slyther as a pet when she found herself on the planet. She named it Doctor and allowed it to eat Thals. (PROSE: Postcards from the Universe)

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 the end of the fourth episode, "The End of Tomorrow", and the start of the fifth episode, "The Waking Ally".
 * 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 of a minisode featuring a Slyther's "adventures" called Sid's Date.
 * 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 at a completely different form of adversary.
 * The description of the Slyther in Terrance Dicks's novelisation of The Dalek Invasion of Earth as being "a vast lumpy blob of a body, powerful flailing tentacles, two tiny deep-set eyes shining with malice..." (quote, page 101) is actually that of a Mire Beast from TV: The Chase. It would appear that the BBC sent Dicks the wrong reference photograph.