Scorpions are a species of arachnid, distant relatives of spiders, and are amongst the most recognizable of their breed in the world, thanks to their long, articulated tails with their venomous stings and their bulbous, crab-like claws. Although for the most part inoffensive - the vast majority of scorpions have venom no more toxic to humans than a bee's sting - their startling appearance and visibly well-armed nature has cemented them in the hall of fame as go-to scary bugs.
Scorpions appear in many fantasy settings, invariably falling into one of two molds; a normal-sized scorpion with incredibly toxic venom (and sometimes a propensity to swarm), or a gargantuan monster big enough to hunt people. Both kinds of scorpion may appear in the same setting.
In more science-fiction settings, "Scorpion" is a popular name for a quick-moving, hard-hitting vehicle or mecha.
In Warhammer 40000
The Scorpion is the quintessential Eldar super-heavy, and like its Imperial counterpart, the Baneblade (and now, there's the hover super-heavy Astraeus), the Scorpion spawned multiple children using its chassis to mount their weapons. It is named after a mythical beast from Eldar mythology, and as with most Eldar vehicle names that coincidentally translate to the names of real life human animals, it is never really explained in detail. Scorpions are used to spearhead armored assaults and as long-range fire support for raiding forces, where their devastating anti-tank firepower has earned them nicknames among the Imperial Guard like Grave-maker or Death's Sled.
The armour protecting a Scorpion is unnaturally resistant even by wraithbone standards, capable of somehow deflecting multiple Lascannon blasts like a giant mirror. However, its main defence besides speed and agility comes in the form of the standard issue Titan-grade Holo-field which refracts the Scorpion's image in the visible spectrum and completely fucks with scanners and other tracking equipment attempting to lock on to the craft, resulting in plenty of missed shots and the company leader losing his shit while the Elf piloting it runs away with a smug grin.
The Scorpion's primary armament are a pair of massive Pulsars mounted on a traversing turret. These weapons are an OCP to Imperial Titans, causing them to buckle under a gigantic version of a seizure and can slice a Leman Russ Battle Tank in half with a single shot, and even during the most demanding of maneuvers, advanced sensory and targeting equipment ensures lethal accuracy. Among all the super-heavies fielded by the Eldar, only the Scorpion has the ability to move easily about a craftworld without problems. Its secondary armament comes in the form of a Bright Lance, Eldar Missile Launcher, Scatter Laser, or Starcannon, while additional upgrades come in the form of Star Engines and Vectored Engines.