Headcrab

Headcrabs are cat-sized, crab-like creatures. They come from Xen, but there are some sub-species that don't live 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 muck like a 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 and striking from behind. Fast headcrabs are never 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 ordinary headcrabs. Like the fast headcrab, they`re a new species, complimenting their swift brethren nicely.

Headcrab infected Hosts
Once a headcrab gets a firm grip on a victims head, it will infect and take over the new hosts 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 has been removed, revealing their internal organs. Ordinary zombies are slow moving, so they can be killed at long range without much worry.

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. With these abilities, they are more deadly then the ordinary zombies.

Poison Zombie
Poison zombies are the strongest of all zombies. They can carry two another poison headcrabs on their back and can withstand many gunshots. Like the ordinary zombies, there are very slow.

Source: Half life (2)