Archetypes can be defined as things that are used as typical examples or in other words, unoriginal ideas that are widely used. We usually refer to them as cliche's.
In the horror genre, other archetypes that I can think is just before the movie ends, the villain/ghost/monster appears indicating that it's not yet the end of the story for our antagonists. Archetypes are used because it has been a tradition or trademark of, in this case, horror films.