Only one Gonarch specimen has been encountered by Dr. Freeman and is many times larger than an average headcrab, nearing the size of an adult elephant. Its age was unknown, as was the biological process necessary for it to reach this form, but it was extremely strong and dangerous.
The Gonarch is much more crab-like than normal headcrabs, with distinct crustacean chitin covering its body and legs. It has no clear sensory organs and its defining feature is a large sac of flesh hanging beneath it. This sac is the Gonarch's reproductive organ and continuously spawns infant headcrabs, even in the middle of battle.
When Freeman entered the Gonarch's lair, it ferociously lashed out and attempted to kill him by charging at him and spawning offspring to attack. The headcrab babies were weak and could not cause much damage individually, but the Gonarch would constantly spawn them in great numbers. The Gonarch's armoured carapace was highly resilient to all of Freeman's weapons and though the reproductive sac proved to be an obvious weak spot, it still absorbed large amounts of ammo from Freeman's arsenal before the creature was finally slain. Upon death, the Gonarch violently exploded and most of its offspring in the vicinity perished with it.
- The name "Gonarch" is a portmanteau of the words "monarch" - meaning royalty such as a king or queen - and "gonad", another word for genitalia. This name fits the creature's status as the queen/matriarch of the headcrab species and its sac closely resembles a giant testicle.
- The Gonarch was also nicknamed "Big Momma" by the development staff at Valve.
- While Gonarchs do not appear in Half-Life 2, the Combine are able to breed headcrabs as biological weapons. A model of a Gonarch breeding pod suspended by mechanical braces was cut from the finished game.