Arachnid Alien

From Alien Species

One of the most interesting Xenomorph breeds yet known, Arachnid Aliens are in fact Facehuggers that hadn't lashed on to a host. The "egg" inside them starts breaking down after a while, fusing with the facehugger, thus creating the Arachnid Aliens.

They basically look like realy big, black skinned spiders with long, bony scorpian tails that they can use to impale their victims. Unlike other Xenomorph breeds, Arachnids can't swim. This is posibly because their bodies are not designed to do so and/or that the Facehuggers' "breathing sacks" had mutated into lungs, keeping it alive instead of its non existant host.