

The zombies it creates are stripped of most of their flesh and muscle. It does not have a beak like the other varieties, instead using the sharp talons at the ends of its legs to latch to hosts. Its skin is slightly lighter, and it has long, spider-like legs that allow it to move much faster and climb on walls (first demonstrated in Half-Life 2 in Ravenholm). The fast headcrab is a faster, more spider-like version of the ordinary headcrab. It was explained by the Half Life team that the creature was based on the idea of “What if we put a giant testicle on a 20-foot-tall armored spider?” which may be its name sake, a combination of gonad and monarch. Unlike other types of headcrabs, the Gonarch lacks a mouth on its underside, suggesting it is unable to zombify a host. The Gonarch appears to act as a sort of hive queen, endlessly spawning infant headcrabs from a bulbous fleshy sack located on its underside. The Gonarch is a massive, heavily armored version of a headcrab that appears only once as a boss fight towards the end of Half-Life in Xen.

Headcrab zombies also receive these variations in Half-Life 2. While the original Half-Life only contains one type of headcrab, Half-Life 2 introduced two major variations: fast headcrabs and poison headcrabs. Bullsquids, Vortigaunts, barnacles and antlions will all eat headcrabs and Vortigaunts can be seen cooking them in several locations in-game. The games also establish that while headcrabs are parasites that prey on humans, they are also the prey of the creatures of their homeworld. Headcrabs and headcrab zombies die slowly when they catch fire. The converted humans are more resilient than an ordinary human would be and inherit the headcrab's resilience toward toxic and radioactive materials. Headcrabs seek out larger human hosts, which are converted into zombie-like mutants that attack any living lifeform nearby. However, they can leap long distances and heights. They are also relatively slow-moving and their attacks inflict very little damage. Physically, headcrabs are frail: a few bullets or a single strike from the player's melee weapon being sufficient to dispatch them. Under the headcrab's body is a large rounded mouth surrounded by mangled, rigid flesh with a sharp claw-like beak. Their pair of large frontal claws are for attacking, and as additional support when standing still. The common headcrab variant has rounded bodies with four legs for movement, two of which are long clawed legs at the front and two stubby legs at the back. Headcrabs are depicted as parasitic lifeforms roughly 2 feet (0.61 m) long.
