Headcrab

Headcrabs are omnivorous, parasitic alien organisms. These creatures have made appearances in a majority of releases on titles based on the Half-Life universe. Thanks to this, and the fact that they spawn frequently through the course of the story line, these little creatures are ensured to engrave in the head of users who play one or more Half-Life titles. This has lead the community into considering the Headcrab as one of the most iconic creatures in the Half-Life universe.

In When the Cold Breeze Blows Away, it will be first appeared that they are appeared in Xen but also appeared in somewhere universes if not only located in Half-Life universe.

They are neutral enemies due to their parasitic alien which attack anyone or turn anyone to be zombies unless survivors try attack them.