An Alien is a type of Doodad in Broforce. They are introduced, obviously, in the Alien Infestation update. They play an important role in the Campaign, as they are fighting against everyone to conquer territory. They are the second type of threat encountered. After Terrorists(Mooks) and before Demons. When they are messing up, the Infestation meter will appear.
Facehuggers are the initial larval stage of the Xenomorph. Like fully-grown Xenomorphs, facehuggers are extremely mobile and easy to kill. Facehuggers are responsible for the creation of more Xenomorphs by way of impregnation, and attack both Bros and mooks.
Screechers are facehugger variants that use a kamikaze-type attack to dispatch Bros and mooks alike, by rolling into their enemies before exploding in a shower of acid.
Banelings are aerial variants of the Screecher that also use a kamikaze-type attack to kill their enemies - rather than roll, Banelings dive-bomb from the air onto their enemies before exploding in a shower of acid. Like Airstrikes, when they are aiming a green straight line and a green cross can be seen, to warm the Bro about their impending attack. Sometimes they will aim erratically and simply carsh into something nearby instead of landing at the cross.