From WoWWiki

A zergling is a small, fast, maiming melee kill-machine that's the main disposable unit of the Zerg in Starcraft. They have four legs, sharp teeth, and two scythed arms that fold under their front legs while running but can rear up when attacking.

It found its way into World of Warcraft as a Collector's Edition vanity non-combat pet. In Warcraft III it was an inside joke (along with the Hydralisk, Marine, and Space Fel Orc) within the World Editor, but frequently finding their way into user-made maps.

In WoW, the popular 'Zergling rush' strategy is adapted to mean attacking a player or mob with a far bigger group.

[edit] As a non-combat pet

See [Zergling Leash].