The mecha hydralisk den is a structure available to Egon Stetmann in Co-op Missions. It functions much the same as a normal hydralisk den, with the added bonus of allowing Stetmann to respawn defeated hydralisks from scrap parts. They do not require creep to be constructed. The den can evolve into a mecha lurker den.
Mecha hydralisk dens will burn down with put below 1/4th health like terran structures, but can be repaired by mecha drones.