Alien is a masterfully constructed horror film about unknown and escalating threats. Escalation of danger is one of my favorite things to see in a story, whether it takes place in a single scene (Spielberg executes this wonderfully in the opening of Raiders and in the first T-Rex attack in Jurassic Park), or over the course of the entire work. Alien is an exceptional example of the nature of the threat becoming steadily worse over the course of the film.
In many other stories–many of which I love–the animal, monster, demon, killer, etc. is completely or largely known from the beginning of the story, or even before that. Even if you hadn’t read the novel beforehand, the poster and previews for Jaws would let you know that the threat is a massive great white shark. Despite not seeing the shark for much of the movie, the only true and slight unknown is just how big the shark is, which we effectively learn when we hear of its “bite radius,” even before we get our first glimpse of it.