head crab

The head crabs attach to the head of a host, and in some way use that host to form a fully-developed member of the species, which is generally much larger and scarier than the head crab.

Became famous under this name in Valve's game Half-Life. Half-Life's head crabs permanently attach to the host's head and turn it into a flesh-eating zombie. They prefer to dine on scientists, marines, and police officers. The original head crabs may be from the movie Alien.

These head crabs put the host into a coma after incubating it with an egg. The egg develops into a baby alien which then tears through the chest of its host, and perhaps eats it as well. There are several known instances, however, where the head crab has been removed, or detached on its own. The host continued life as normal until one day, quite suddenly, an alien ripped through his chest.

Ouch.

The name normally used for the little beasts from Alien is "facehugger".

In Halo, a small creature that attempts to attach itself to the Master Chief. If the Master Chief's shields are depleted, the creature will succeed and drain health at an alarming rate (especially when multiple are attached). Having a swarm of these creatures attached to you is a sign that you are about to die. Fortunately, a single shot from any weapon will cause a creature to explode, possible fratricidally if others are nearby.

In the Metroid games, a creature about the size of a human head that attaches itself to Samus and rapidly drains energy, growing larger. Can be removed by entering morph ball mode and dropping a bomb. A good preventative measure is freezing one with an ice-based attack before it can attach itself.