The Fast Headcrab is a subspecies of Headcrab that appears in Half-Life 2 and its episodic sequels. It is unknown if they are a natural evolutionary offshoot of the standard headcrabs as this type was not encountered by Gordon Freeman when he visited Xen. This breed has only appeared since the Combine invasion of Earth and is suspected to have been created by the Combine through genetic manipulation for use as a biological weapon.
At first glance, the Fast Headcrab can be easily mistaken for an ordinary headcrab, but upon closer inspection the physical differences are quite apparent. The legs of the creature are thinner and end in sharp talons, and there is a complete lack of mandibles and beak. The body of the creature is also marginally smaller and covered with a number of small, wart-like growths.
As the name suggests, Fast Headcrabs move much faster than the other headcrab variants, taking less time to close the gap between them and their prey. They attack as other headcrabs do by leaping at prey and latching on to their heads, but since Fast Headcrabs do not have beaks, it is unknown how exactly they take control of would-be hosts. It is believed that they use the talons on their legs to puncture deep into their victim's flesh and grasp the spinal cord and manipulate the body that way rather than seizing the neo cortex.
Fast Headcrabs are just as physically weak as standard headcrabs and it does not take much to kill them, though their speed makes it more difficult to target them. Automatic weapons are the best choice for dispatching these creatures or grenades if threatened by a large pack of them.