Notable authors that bridged the idea of aliens and horror are H.P. Lovecraft and Ray Bradbury. Some of the most influential early films include The Day the Early Stood Still (1951) and The Thing from Another World (1951). But of course there were many others, from Invasion of the Saucer-Men (1957) to Planet of the Vampires (1965)
By the 1970s and ’80s we had films like the Alien and Predator franchises giving new dimensions to alien horror.