Wellp, a lot of people conflate horror, gore, thriller, and sci-fi, probably because X movies often contain bits of all of them.
Eg, “The Monolith Monsters” might be considered horror, because it’s got villians but which are made of stone, and not alive, per se. But it’s also mainly sci-fi, because there was no malevolence, just space-rocks that sucked in water and would grow to monstrous proportions, crack into pieces, and spread like weeds.
Jason/Freddy/Michael/Pinhead would probably be horror and gore.
Creep-in-the-house or other stalker flicks, more thrillers than horror, unless there’s a gore factor in which case they’d also be lumped into horror.
Old-timey flicks like Mummy/Frankie/Wolfie… horror, I imagine, but few if any jump-scares that seem to permeate “modern” horror.
“The Hound of the Baskervilles”, implicit horror, later revealed to be purely of earthly origin, so… thriller?
You left out a major factor in the popularity of modern horror/slasher movies, the frequent need of the young actresses to shower, thank goodness showering didn’t figure into the roles of Boris Karloff and Bela Lugosi.
Depends on movie.
Evil dead with memorable Bruce Campbell or Shaun of the dead sure.
Alien, The shining, The thing, hilarious Dead don’t die, From Dusk Till Dawn, Grindhouse…
Mike