Headcrab

Headcrabs are cat-sized, crab-like (or perhaps tick-like) creatures. They come from Xen, but there are some sub-species that are not indigenous to there. Rather, most of these sub-species live on Earth. All headcrabs have the ability to infect hosts.

Ordinary Headcrabs
The most common headcrab. This variant has pale tan skin and dark red legs. When threatened it will lift up its front legs much like some species of spider. It attacks by leaping at an enemies head and biting with its beak like jaws. It is also weakest species of headcrab, so a whack with a blunt object will usually bring it down.

Fast Headcrabs
As their name implies, these headcrabs have longer legs, increasing their speed and making them difficult to hit at close range. This headcrab variety also employ better tactics, weaving around their opponent to strike from behind. Fast headcrabs have never been encountered on Xen, only on Earth, suggesting they may have been created by the Combine purely for use as biological weaponry.

Poison Headcrab
Poison headcrabs are stronger then the other headcrabs. They have black skin and many small spikes on their body. Instead of biting and damaging their opponents, these headcrabs deliver a potent neurotoxin to their victims, making them easy prey for other enemies. They are very slow, more so than even ordinary headcrabs. Like the fast headcrab, they`re a new subspecies, complimenting their swift brethren nicely.

Headcrab Infected Hosts
Once a headcrab gets a firm grip on a victim's head, it will infect and take over the new host's nervous system, turning it into a headcrab zombie. The type of headcrab determines the type of zombie, each having different abilities.

Ordinary Zombies
These zombies are stronger than humans and swipe at enemies with their elongated claws. Their chest and ribcage have been removed, revealing their internal organs. Ordinary zombies are slow moving, so they can be killed at long range without much worry.

Gonome
The Gonome is evolved from the standard zombie. It can toss gobs of explosive slime at players, and features longer claws. It sits somewhere between normal and fast zombies in terms of speed.

Fast Zombies
To increase their speed and mobility, fast zombies have done away with their skin and organs. They are very fast, bounding on all fours to their target, and can easily jump 2 meters (6 and a half feet). With these abilities, they are obviously more deadly then the ordinary zombies.

Poison Zombie
Poison zombies are the strongest of all zombies. They can carry two other poison headcrabs on their back, which they throw at potentiial hosts. They can withstand many gunshots. Like the ordinary zombies, there are very slow.

Cameos

 * In Metal Slug 6, a zombie-creating creature that attaches to the head of its victims is featured.

Source: Half life (2)