ExtraTerrorstrial
Article by Bindi Lavelle

The unopened door, the unknown, these ideas feed our curiosity and our fear. And the greatest unknown there is, is space. In space, not only can no one hear you scream, but there is a plethora of nasty creatures waiting to take over Earth and kill all the humans. In the evil extraterrestrial club, there are some stand out aliens, whose most notable feature is that they are creepy as hell.
Take for instance the Body Snatchers, from the 1950s film of the same name and the later remake. These slippery customers do pretty much what their name suggests, they snatch bodies. What's so scary you ask? Well, they are parasites that take over their host's body and acquire their memories so you can't tell who is an alien.
The Body Snatchers, however have nothing on The Borg from Star Trek for sheer efficiency in their destruction; The Borg assimilate all the technology and biological material of a planet into their collective. The Borg have a hive mind and operate solely for the collective, kind of like evil bees.
Arguably, the most iconic of these scary aliens is the H.R Giger designed Aliens, from the Aliens (and Predator) film franchise. The combination of savagery and sexuality is what makes the terror factor; pornographic curves frame some ferocious teeth and claws, not to forget that they spit acid too.
Also in terms of design, there is John Carpenter's remake of The Thing, an extraterrestrial that can morph into the shape of anything has a taste of, creepy in a similar way to the Body Snatches, but with a touch more gore and violence.
And then there is The Blob, which is a gross looking blob of ooze that is not only slimy and icky, but absorbs its prey like a sponge, a sponge of doom. It's defining quality is that it is a blob, which in my op pinion is very nefarious indeed.
Horror is a genre inhabited by a vast variety of villains, from all manner of origins and extraterrestrials of all descriptions feed our imagination and fears.