A really good scary alien species is found in Peter Watt's *very* EP-esque "Blindsight". Sometimes the truly frightening thing about aliens is what they imply about the world or humanity, rather than that they are scary in themselves. One (rather cerebral) variant of this is the Wang Carpets of Greg Egan's "Diaspora" (and the short story called Wang's Carpets). My own most successful creepy aliens were the Dragons, a global biological internet between bacteria in the Earth's crust. The Dragons seeded life on Earth 4 billion years ago, added backdoors to DNA so that they could use cells as processors and then set up the lithoautothropic ecosystem as a slow but vast biological internet. Some cells mutated and became a biosphere on the surface, but to the Dragon civilization that was of little concern. Until they ran into some kind of trouble and needed to escape. At that point they began to upload themselves into the biosphere using their root access to all terrestrial life. A simple plasmid signal and any biomass - coral reefs, jungles, algal blooms, humans - could be turned into whatever they needed. So they set out to remake the biosphere into a dracosphere. See this and this for more information. I'm also rather happy with the To’ul’h that I introduced in Orion's Arm (and was subsequently developed by others). They are jet-black, look like something between bats and vampyrotheutis squid, have mandibles and various echinoderm features, and live under the dark clouds on a Venus-like planet. Seriously scary when first met. But underneath these superficial differences they are pretty humanoid in mindset. Which of course doesn't preclude ordinary humanoid treachery or stupidity.

There are already the TITANS and all the horrors they have unleashed on transhumanity. I don't think you need to create an alien species like the Dragons that where laying in wait. That being said a bioware-addicted race that needed new biological stock for their alien bioware augmentations would be a sufficiently evil race to make contact with (through a gate of course). Or even more sinister if you took the idea from the movie "I Come in Peace ( http://en.wikipedia.org/wiki/I_Come_in_Peace ), but take out the alien police (ie you come across an mob run alien species that harvest pure adrenaline from "lesser" species to sell as a drug on some alien black market) that would be horrific in its own right. I think in EP any horrific alien encounter should be through the gates, or a first contact in the Oort Cloud ( http://en.wikipedia.org/wiki/Oort_cloud ).

I quite agree with the fearfulness of a species that one cannot communicate with. Either because it is too alien, or because it is instinctual. Karl Schroeder has some ideas about this in "Permanence", where one of the chilling ideas in the setting is that intelligent species tend to evolve towards post-intelligence. Once they have mastered their own situation all intelligent species tend to create environments they are perfectly adapted to, or remake themselves to fit their environment. At that point there is no longer any pressure to remain intelligent, and they tend to evolve into something else. Robin Hanson has a scary paper, "Burning the cosmic commons" where he gives an evolutionary argument why a species spreading across interstellar space will evolve towards what is essentially a locust swarm using all available resources to spread. His argument (in a very simplified form) is that in the long run the descendants of colonies that colonized a lot will be the most numerous. If the tendency to colonize is inherited by daughter colonies with some variation, then there will be an evolutionary pressure towards extreme colonization. So maybe all species that go interstellar will turn into replicator swarms... unless the ETI stops them.

No, it doesn't. Take Babylon 5 for example, neither the Shadows nor the Vorlon are evil, even if they could very well wipe out all living species in their games.

Another source of horror are the primitive nothingness and force of "natural" things. Terrorific things. An earthquake, a typhoon. All of those relate to one important element of the terror feeling: the feeling of hopelessness and helplessness. Doesn't matter where you go, or what you attempt, it will be over it and destroy you. There is no sense in even trying to fight it. All that's been said here ties into probably what makes the most terrorific movie of all times (In my opinion): Alien. The Alien film is set in a place where there is no escape (opening the door to helplesness), and is always dark and oppressive. You don't get to really see it all, there are always shadows and turns, and mysterious places where things could hide. And yet, it is well lit, and camaera doesn't focus on those elements, so they don't "jump to your eye", they are in the background, causing rising paranoia for apparently no reason. Music plays in this too, for even in the quiet and tranquil parts, it always has strange hints in it, making it a little unsettling for apparently no reason (the Silent Hill OSTs are always very good for this too). A thing to take into account is that it is a "safe" location where it takes place: a ship in space where nothing "magical" can happen. A techonological place, where people feel safe, because they feel they control the universe around them through that technology. A ship they've known for long. And even the look of the ship represents that apparently non-notorious scenery: even though it has shadows, they are not formed by odd turns, but by pipes and other elements one would expect to be present in a mining barge. So, at first, they all feel safe and tranquil, and both things will be slowly taken away from them, increasing the feeling of loss and lost control. And taking control from the characters is a key element for making them helpless. Second, the alien itself is always a mystery. It looks like an egg, a spider, a worm, and finally the alien itself. Each time you seem to know what he can do, or get to see him, he elvolves into something completely different and all your knowledge of it is wasted, useless. And all of its forms are perfectly adapted to the situation, and completely lethal. You, again, are helpless, and hope shines one step further away. Third, you can't understand it. It won't communicate. And yet, it is ancient, it was made a prisoner by a more ancient (and probably advanced) race, to who's fall it survived (an Iktomi, maybe?). And yet, even though it is ancient and un-understandable, it seems primitive and primal: it feeds. It knows no fear, but it seems to plan and act intelligently, even if not by human-logical parameters. It is a force of nature, destruction itself, a reaper without morality, nor good nor bad, simply hungry. Finally, it seems to feed on terror as much as it does with flesh. It could have fallen on the center of the group and devoured them all at the same time... and instead, it hunts them one by one, when they are alone, when they are afraid. Leaving Ripley increasingly alone, increasingly chased, hunted, by something she even doesn't know what it is. In the end, they are not fighting an alien at all, they are fighting death itself... something all humans know will triumph in the end.