Gonarch

Appearing for the first and last time in the original Half-Life, the Gonarch, or 'Big Momma' as nicknamed by the developers of the game, is the mother of all headcrabs. It appears to have no partner, so it may be hemaphroditic. Its anatomy is similar to that of the headcrab, with an apparent armour-like exoskeleton, four spindly legs, but it is also hued a mottled blue, with a sac protruding from its belly, from which underdeveloped baby headcrabs constantly spawn. The Gonarch has no visible head, eyes or mouth. The mother headcrab is viciously territorial, and is remarkably speedy. One can tell when the Gonarch is approaching when they hear a horse-like galloping sound, or when it unleashes an elephant-like cry. The animal will attempt to crush intruders with its sharp legs. It is extremely difficult to kill, taking almost everything in Gordon Freeman's armoury.