Arachnid Aliens are Facehuggers that never latched onto a host.
The Xenomorph embryo inside of them starts to break down.
The Facehugger and the embryo are fused together.
They look like large black spiders.
They have long, bony scorpion tails with which to impale their victims.
Unlike other Xenomorph breeds, Arachnid Xenomorphs can't swim. This is possibly because their bodies are not designed to do so, and-or that the Facehugger's 'breathing sacs' had mutated into lungs, keeping it alive instead of its non-existent host.
An arachnid Alien is seen in a comic book among many other xenomophs and when they break out you see it attacking the androids.