If you Google "Merriam Webster classic horror definition," a giant guy with bolts in his neck will sneak up behind you and strangle you…because Frankenstein defines classic horror.
But what does "classic horror" mean? Well, of course, it means monsters—the Wolf Man, Dracula, the Creature from the Black Lagoon.
But horror also means the sneaking suspicion that maybe, those monsters, over there, aren't so different from these poor humans over here. These days, you see that with zombies, who are always turning other poor, normal souls into ravenous flesh-eaters. Everybody, in fact, is a potential ravenous flesh-eater. It's the zombie way.
You can see the outlines of that way back in Frankenstein. The monster, after all, is made out of dead bodies, just like today's zombies. And our monster is human in other ways, too. He gets bullied by Fritz, he wants to play with Maria. He's confused and scared—emotions even non-monsters can recognize.
Horror is about scaring you, no doubt. But it's also about scaring you by showing you that you're scary too. In Frankenstein, you fear the monster and you identify with that same poor monster. We don't know whether to fear the unknown or to fear the innocent-seeming villagers.
And there's nothing more horrific than that uncertainty.