Fast Headcrabs and Fast Zombies are found in Half-Life 2. Fast Headcrabs are what you see in the Ravenholm chapter and so on. Fast Headcrabs are quicker than normal ones but have same health and do same damage. Fast Zombies are Fast Headcrab's victum. Fast Zombies are quicker but have same health, but do less damage. Like regular Zombie, aim for the head. Plus Fast Zombies also come in torsos (starting in Episode Two).
Fast Headcrabs and Fast Zombies are quicker than normal ones. Fast Headcrabs have more spiny legs than normal Headcrabs, Fast Zombies look like skelatons.