Zerg

The Zerg Swarm are the primary villain faction the RTS game Starcraft. They are very similar to the Tyranids from Warhammer 40,000, being a collection of assimilated, genetically enhanced species controlled by a hive mind, which seeks to assimilate new species. The plot depicts them as the most powerful faction because of a massive numerical advantage over their enemies. The size of swarm, and exactly how strong the Zerg units are plotwise is highly ambiguous, since they're from a computer game.

Gameplaywise, the Zerg focus on fast and inexpensive units. Starcraft's popularity has resulted in people labeling fast, cheap, basic units in other RTS games as Zerglings (named after a Zerg unit that fits that description).