I think Alien IS a horror movie. It follows many of the staple "horror" elements: making the audience jump (from the facehugger to the Alien snatching Dallas and Parker to the final scene in the escape shuttle), some blood and guts (mostly Kane getting the male version of a C-section), the dwindling cast of characters (from 7 and 1 cat to 1 and 1 cat) (the animal never gets it) in complete isolation (spaceship fifty years from Earth might as well be a lost cabin in the woods)... I think it's a very special horror movie.

Aliens, of course, is not. That's action all the way down. And that's all good.