Horror movies and sequels go together like PB and J. Produce one relatively successful horror movie, garner even a modest audience, and it's more or less a given that further films will follow.
And we're not just talking the big cheeses like Halloween, Friday the 13th, Nightmare on Elm Street, Scream, Saw, Paranormal Activity, The Conjuring. Very often there are horror movies that the wider audience has never heard of that have still spawned follow-ups: The Dentist, Killjoy, Grave Encounters... we could go on.
Oftentimes, sequels are made for horror movies that wouldn't appear to lend themselves to a franchise formula at all, and we wind up with little more than hollow facsimiles of what came before. This naturally lends weight to the argument that, as Randy so metatextually declares in Scream 2, sequels are by definition alone inferior films.
Even so, there are instances when a movie would seem to have franchise written all over it, yet no sequel ever comes. Sure, we might count ourselves lucky under these circumstances, grateful that we haven't had to see another once-successful formula gradually watered down to nothing - but we also can't help but wonder what might have been.
Are we necessarily any poorer for not having follow-up films to these 12 horror movies? That's open to debate - but in each instance, it isn't hard to see how the filmmakers could have given us more.