Horror fans, as well as Science Fiction fans and Fantasy fans, are nuts about the mythology of their genres. I find this all too appropriate given that these are really sibling genres all birthed from the same mother: Mythology herself.
Other genres such as Action, Adventure, or Romance are all related in the sense that they are all fiction, but Horror and her younger sisters Fantasy and Sci-Fi (who I swear are twins) are different from the others, yet share a connection evident in their fandoms. Surely any genre can be crossed with any other genre, such as a Horror-Adventure-Comedy (Army of Darkness, anyone?) but that is one genre taking on the characteristics of another for purposes of telling the story. At their cores, the Sisters (which I will now call Horror, Sci-Fi and Fantasy as a group) are the true heirs of their respected mother, Mythology. The fans of the Sisters are obsessed with minutia and continuity. They are obsessed with the mythology of their fandoms. I don't mean the mythological underpinnings, which is a legitimate concept in much of the works of the Sisters, but the mythologies that develop as part of the genre works themselves. This is evident when we look at the blogs, the forums, the fanfics (love them or hate them, fanfiction works are some of the purest expressions of the mythologies of the genres), and the fan-films found all over the internet. Prior to the internet there were newsletters, conventions, clubs, and simple meetings where the mythologies were developed and shared. For the fans of the Sisters the mythology is as important as the works themselves.
Look at the internet backlash when a new installment, episode, book, what have you, comes out and breaks the understood fan mythology. Look at Jar Jar Binks, damn you!
Need I say it? MIDICHLORIANS!
An excellent example of this can be seen in the Friday the 13th franchise. The casual viewer of the Friday films can tell you Jason is the killer of the series. Most casual viewers can probably also tell you his last name. The hardcore fan will tell you that Jason is the only son of Pamela and Elias Voorhees, that he had a half-sister named Diana (daughter of Elias) that he was not the killer in the first and fifth films and that he got his hockey mask in the 3rd film. The really hardcore fans will tell you even more, having gleaned information from interviews, screenplays, behind the scenes cutting room footage, issues of Fangoria, and dozens of spin-off media.
It's all part of the mythology of the franchise and the hardcore fan is interested in it. Indeed the hardcore fan becomes offended when something breaks the "canon" of the franchise and will go to great lengths to establish how this occurred.
They do it with Star Trek. They do it with Star Wars. They do it with Dragonlance, Lord of the Rings, and all manner of genre work that collective are the darling daughters of Mythology herself.
Because Mythology was the first horror movie.
Yes, I know it is supposed to explain origins, culturally speaking, and what Joseph Campbell said about it, but these primordial monsters that have haunted us for generations come from Mythology. It's the source of our nightmares, bless it.
Which is why, I believe, we have documentaries about how horror films have impacted society and reflected society. We love, us fans, to delve into the mythology of our fandom. You can't get this kind of mythology from an Action flick. Not this level of commitment.
Anecdotal Evidence: When I was young the older kids I sometimes hung out with loved A Nightmare On Elm Street. In the 80s Freddy Krueger was cool. I learned from an authority on the subject (when you are 13 a 17 year old girl is an authority on horror movies, trust me) that, as she put it, Nightmare 2 sucked and was not part of the story. Just skip it, was her advice. It was A Nightmare On Elm Street and A Nightmare On Elm Street 3: The Dream Warriors. She was not the only person to tell me this, by the way. I suspect Don Dokken would agree.
And if you watch the films from start to finish, back-to-bloody-back you will see that Nightmare 2 just doesn't fit. Freddy's powers are weird in that one, he seems to be trying new tricks. The mythology is wrong. Nightmare 2 does nothing to advance the mythology of the series, but Dream Warriors? Ah, now that is proper mythological development. Many of us like to pretend Nightmare 2 doesn't exist, but Nightmare 3 makes up for it. Firstly it has Nancy. Final girl from Nightmare 1. That's a sequel. Secondly it expands upon Freddy's dream powers and we have the kids who also have dream powers. We are seeing a mythological development that is logical for the franchise. Note that mythology does not have to be logical in the real world sense, but it does need to be logical to the IP itself. All films after 3 followed the logical mythological development to a reboot, which kept with the established mythology. That is how you win the fandom's love and admiration. Well, and respect, I suppose.
Much like many people like to pretend Halloween III: Season of the Witch did not happen because it was not part of the Myers cycle. Well TOUGH SHIT BECAUSE IT DID HAPPEN, SUNSHINE! And it's a damn fine work, so there. Yet it does suffer from breaking the mythology of the previous films and that was rectified in the following 5 films as well. And then Rob Zombie happened. Dammit.
And that is coming from someone who likes Zombie's work. Except Lords of Salem. That was pig shit.
That's my argument about the Mythology of Horror then. Not the mythology underpinning horror, or why we are afraid, but the mythology of the franchises and works themselves, for which the true fans hunger and will develop.
I could be wrong. It happens.