Spider Dalek

Spider Daleks, or Spider-Daleks, were a Dalek variant.

Nature & functionality
The Spider Daleks' casings were slightly larger than normal Dalek drone, and had several legs: some had eight, (PROSE: War of the Daleks) while others had just three. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

They were more manœuvreable than standard Daleks, but more vulnerable at their joints. Giant-sized versions, known as Striders, were ten times larger than the standard Spider Dalek. (PROSE: War of the Daleks)

Up until a certain point, Spider Daleks were rarely deployed as part of armed forces principally composed of ordinary drones. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

Other Spider Daleks
A faction of Daleks from a parallel universe resembled Spider Daleks, and were viewed as abominations by the Daleks of the "main" universe. (COMIC: Fire and Brimstone)

During the Last Great Time War, some of the Skaro Degradations had a spider-like design. These Degradations were also called Spider Daleks. (PROSE: Engines of War)

History
The Spider Daleks were one of Davros' "stranger designs" from an early point in Dalek history. While several accounts indicated these Daleks were successfully created, (PROSE: War of the Daleks,  The Astounding Untold History of the Greatest Enemies of the Universe) one indicated Davros never got the design to work. (PROSE: Father of the Daleks)

After his resurrection and participation in the Imperial-Renegade Dalek Civil War, Davros noticed Spider Daleks among the Dalek Prime's forces. He was pleased to see that they were still in existence. (PROSE: War of the Daleks)

An Imperial Earth Spydrone once captured an aerial photograph of a Dalek invasion fleet. A three-legged Spider Dalek was visible alongside the numerous Grey Daleks. This reportedly puzzled military analysts, as Spider Daleks had never been observed to be deployed alongside normal Dalek forces up until that point. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

Prior to the outbreak of the Last Great Time War, command ship had an "arachnid chassis" in its archive of Dalek casings dating back to Davros' earliest experiments. (PROSE: Father of the Daleks)

Behind the scenes
The idea of Spider Daleks originated with the very different spider-like Daleks of the Leekley Bible, which Fire and Brimstone would eventually confirm as Daleks from another universe, showing them coming face-to-face with the Dalek Hive. In neither story were they, actually, referred to as "Spider Daleks". The depiction of the Spider Daleks in Dalek: The Astounding Untold History of the Greatest Enemies of the Universe is based off of Daryl Joyce's artwork depicting The Time of My Life in DWMSE 5.

Spider Dalek is one of Dalek enemies used in the Doctor Who: Legacy mobile game.