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
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 Spider Dalek was a new version of Dalek which might have appeared in a proposed revival of Doctor Who developed during the mid-1990s. (A backdoor pilot for the proposed revival appeared as Doctor Who).
The redesign was intended to take full advantage of the then-new technology of CGI, but bore little resemblance to their predecessors. Concept art depicted these creatures as having six-legged, adaptive casings, from which their mutant controller could emerge, wielding claws and lethal weaponry. A brief screen test of these new all-CGI Daleks was broadcast on television as a trailer of sorts, early into the movie's development, and was later released on DVD alongside the TV movie itself.
In the finished Doctor Who TV movie, however, the Spider Dalek was not featured: the Daleks executing the Master in the prologue are heard, but emphatically not seen. Later merchandising and continuity would establish these Daleks to have been "ordinary" Imperial Daleks of the Dalek Prelature.
Spider Dalek is one of Dalek enemies used in the Doctor Who: Legacy mobile game.