A scorpion is an arthropod with eight legs, belonging to the order Scorpiones in the class Arachnida. This class also contains the spiders, harvestmen, mites, and ticks. There used to be approximately 2000 species of scorpions and could be found widely distributed south of 49° N, except New Zealand and Antarctica.
Scorpions are predators, and possess a powerful set of forelimbs to kill or grasp potential prey. Against larger prey, scorpions will sting them (injecting a powerful venom) until the prey dies. Scorpion venom is also a defense mechanism to deter predators.
Nod symbolism (Tiberium Universe)
Nod's most widespread motif is a black scorpion tail, poised to strike, inside a stylized red quadrilateral. Variations of this symbol appear on banners, the Nod Achievement Medal, and the Black Hand insignia. The Symbol was also placed on their military units such as being painted on the Scorpion Tank.
Additionally the scorpion aesthetic of black colors and a carapace-like superstructure was prominently found in Nod constructed buildings. Nod officers always wore black uniforms, evocative of scorpions although Nod soldiers tended to dress in silver or red; but with black also being prominently found.
Also, throughout the history of the Tiberium Wars, the Temple of Nod has also been deliberately evocative of a scorpion, more so as time went by, with the exception of those inspired by Egyptian and South American Pyramids. Interestingly, the temple has often functioned as a tactical nuclear missile silo and Scorpions are largely immune to radioactive fallout.
The exact meaning of Nod's scorpion motif however is curiously not known. The Scorpion does have plenty of occult symbolic meaning, being afterall one of the signs of the Zodiac used in Astrology. There it's precise is (as everything) disputed but it tends to be associated with force, passion, focus and motion; all properties that Nod would wish to appropriate.
Alternatively the Scorpion reference may be due to the actual scorpion's combination of small size and relatively high destructive potential due to it's venomous tail. The scorpion which is little may be crushed by the bigger creatures but it can regardless sting them. Therefore the scorpion's tail represents the vengeance of the low-ranking proletarian people, which Nod wishes to be perceived as the champions of, against the high-ranking wealthy people such as the GDI.