Horror is one of the most popular genres in film and television. Horror as a genre emerged in the 8th century with the Gothic novel. The genre is believed to have been founded by Horace Walpole, author of Castle of Otranto (1765).
'Alien' works because the design of the Xenomorph creature is so fascinatingly hideous. Director Ridley Scott reached out to a famous artist to get the design in the film.
Truth is scarier than fiction. Some of the most popular Hollywood movies are actually based on real life crimes, including one of the most famous horror movies ever made!