Headcrab

Headcrabs are cat-sized, crab liked creatures. They`re come from Xen, but there are some sub-spezies who aren`t live there, but on earth. All headcrabs have the ability to infect hosts.

Ordinary Headcrabs
A red and white, slow walking creature. Attacks with his claws. This is the most common and also weakest spezies of the headcrabs.

Fast Headcrabs
This headcrabs have longer legs, so they can run very fast. They don`t exist on Xen, but on Earth.

Poison Headcrab
Poison headcrabs are stronger then the other headcrabs. They have black skin and many small spikes on their body. Poison headcrabs infect their enemies with a strong poison. They are still slow, like the ordinary headcrabs. Like the fast headcrab, they`re a new species.

Ordinary Zombies
These zombies are stronger than humans and have claws on their fingers. They`re body is ripped out. Ordinary zombies are very slow, so they can easily killed with guns.

Fast Zombies
They look like corpses, without skin and fat. They are very fast and can jump easily 2 meters. With this abilities, there 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)