From what I've seen alien zombies they are usually caused by small alien lifeforms that latch onto people and use them for food and transport.
Example 1: The Flood
Initial form:
These creatures are fairly small, around the size of a football. But they are far more dangerous than getting pulled away at the last minute of a kick. They travel in large groups and are excelent jumpers. They use these qualities to swarm a victim and latch onto it. They will proceed to burrow into the victim and infect it. At this point the victim has died but soon reanimates under control of the Flood creature. The host is used for food, transport and creating spores used for reproduction. The best way to cambat these creatures is to kill as many as you can as fast as you can. Their bodies at this point are weak and a blast from a shotgun or machine gun can take out a cluster. Single flood creatures are better to be finished off with a close range weapon like a shovel or crowbar. The best time to strike is when they jump at you, but make sure you hit, otherwise you're flood food.
Warrior form:
When a Flood zombie is created it is automatically a force to be reconded with. Although they may appear slow at first glance they are capable of sprinting at high speeds and can jump inhuman heights and distances. Their transformation is not total at this point so they are capable of using tools and weapons. Normally zombies can be killed with a decent blow to the head but these zombies don't use the normal human brain, they are controled by the creature inside of them. Aiming for the chest is a fairly effective strategy. If you dissable a warriors arms then it will have a reduced chance of doing any damage since zombies can't kick and the head is no longer functional and can't bite you.
Carrier form:
The carrier is the last stage of the flood zombie. These creatures are sacs where flood creatures are waiting to hatch. They are fasted walkers than the warriors but can't sprint or jump, so they're easier to outrun. The arms have reverted to tentacles that can't use tools or weapons... they don't really do too much of anything, but that shouldn't be any reason to lower your defences around them. These creatures are filled with natural gasses that explode when the growth sac is hit. The flood creatures inside are not harmed but are propelled in all directions so watch out! If multiple Carriers are standing together and one explodes it will trigger the others in a large chain reaction. This will kill the creatures inside, but it will also decimate everything else around them. They are usually found with other Carriers or Wrriors so the best strategy is to hit one from a long range and hope it will take out all the other for you. This can be acheived with almost any gun or even just a well placed rock.
Example 2: Head Crabs
Head Crab:
These creatures have four clawed arms wich they use to scuttle around, jump up to face level and latch ontothe victims head. It's entire unbderelly is one large mouth that it uses to bite into a victims skull and eat out their brains. The crab proceeds to take over the bodys motor functions.
Zombies:
These creatures are slow but make up for it in sheer strength. They can lift barrels and crates and throw them alarming distances. The main weakness of these creatures is to take out the crab resting on the head. Also finre, fire works pretty good too.
Fast Zombies:
These zombies aren't as strong but are fast and excelent jumpers and climbers. The head is still the weak point but much harder to hit.
Carrier Zombies:
These zombies have no specific quality aside from two or three other headcrabs on its back that it will throw at any human it sees. These are best taken out from long distances. Best to take out the other headcrabs then take out the zombie itself. A grenade or explosive device will take them all out at once and make a satisfying sound.
Those are the main ones I know of. If anyone knows of any other alien zombies post them here. Cuz if we're preparing for zombies we might as well prepare for aliens too.
