Mustn't forget the Chryssalid, either. The Grue is all well and good, but you never actually see it (though you do get eaten by it). The Cryssalid, however. Oh man... just knowing one of those might be around would instantly put me into a "Nuke it from orbit" frame of mind.
Crysalids, as in X-Com's Crysalids, as in I dont care where you are on the ****ing map give it every burst round you have, just kill it, dead, now, I dont care that you have a rocket launcher and it is 4 squares away from you, DDDDIIIIEEEEEEEEEEEEEE. They really should have included a flame thrower for those things, cook it inside that damned armoured shell. They made the demo hell.
about the article I had to lol at the shivan picture.
also what about Duke Nuckem 3D's octobrains?
sure they are tough suckers to kill at times but tbh even in the first game they were only scary in that time when a corridor goes dark and you are locked in with one early on. UT killed the scariness of the Skaarj and by the time Unreal 2 came around they were mass baddies even the big ones were at most minor boss prestige