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
After his resurrection and participation in the Imperial-Renegade Dalek Civil War, Davros noticed Spider Daleks among the Dalek Prime's forces. He recognised them as being a realisation of an old design of his, which had never implemented himself. (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, Davros had several casings of Spider Daleks in his command ship's laboratory, noting that he'd never been able to make the design work. (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".

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