This article contains lore taken from Warcraft III: Reign of Chaos, Warcraft III: The Frozen Throne, the manuals, and/or official bonus maps.
The Hydralisk from Warcraft III.
Powerful Zerg creature...[1]
The Hydralisk is a hidden creature in Chapter 2 of the Night Elf Campaign, Daughters of the Moon.
They are also found in the editor, Warcraft III World Editor.
Story
Tyrande Whisperwind had just been attacked by Archimonde when she used 
[Shadowmeld] to hide herself. She escaped the Doom Guards to reach safety. Along the way, she found the in-game secret of a Hydralisk hidden in the upper right corner of the map behind a layer of trees. The Hydralisk aided Tyrande in fighting past the Undead forces blocking her way.
Trivia
This is a homage to Starcraft.