Gonarch

The Gonarch is the final phase of the Headcrab's life-cycle. It is encountered in a boss battle taking place in one of the last chapters of Half-Life, Gonarch's Lair. Only a single Gonarch is encountered by Gordon Freeman during the game.

The Gonarch can be seen multiple times in Xen.

In-Game
The Gonarch only appears during Chapter 16 as a boss fight taking place in Xen. It has a variety of attacks which include spawning baby headcrabs, launching acid at the player, and charging at the player.

Trivia

 * The Gonarch appears to be a female.
 * Only appears during Chapter 16 as a boss fight.
 * Has 4 main attacks.
 * Can instantly kill you with one hit on respective difficulties.