Zerg Hydralisk from Warcraft III
The Hydralisk is a Zerg unit from StarCraft, where it serves as a ranged attacker. It is included in Warcraft III as a hidden unit in Chapter 2 of the Night Elf campaign, Daughters of the Moon. It is also available in the World Editor, and finds its way into many player-created maps.
Story
Tyrande Whisperwind had just been attacked by Archimonde when she 
[Shadowmeld] to hide herself. She escaped the Doomguards to reach safety. Along the way, she found the in-game secret of a Zerg Hydralisk hidden in the upper right corner of the map behind a layer of trees in the Night Elf Campaign map number 2. The Hydralisk aided Tyrande in fighting past the Undead forces blocking her way.
Origin
How a Hydralisk came to be in Ashenvale is completely unknown. This appears to be a non-canon Easter Egg which has no impact on the actual story. Hydralisks are ranged ground units of the Zerg Swarm, shooting deadly spikes of envenomed bone from their shoulder-joints. They were once peaceful herbivorous creatures called Slothein, but were infested by the Zerg, and transformed into ruthless killers. The more advanced strains can mutate into Lurkers, horrific creatures capable of burrowing and sending out multiple spines from the ground to vanquish the enemies of the Swarm.