The following section contains information from the Warcraft series and is notcanon.
The hydralisk is a an Easter egg unit in Warcraft III. Its attack animation had the shoulder plates opening and releasing a quickly dissipating green mist.
One may be rescued in the Reign of Chaos mission Daughters of the Moon. It is hidden in the forest north of the final undead camp. The trees hiding the unit may be destroyed with rescued ballistae. Hydralisks are also used in the credit animations if the game is beaten in hard difficulty.
The hydralisk was available in Reign of Chaos'World Editor but was disabled in the UI. In the expansion, The Frozen Throne. the unit was fully enabled.