Super 8 (2011) has a monster, but it is not a horror movie. This is not unusual. The Cyclops in The 7th Voyage of Sinbad, Jabba the Hut in Return of the Jedi, and Ernst Stavros Blofeld in You Only Live Twice are all monsters in their own way, but those movies are clearly not horror movies. For a film to qualify as a horror movie, the monster has to be present not just as one character among many, but as the fundamental point of the movie. A horror movie is a film whose primary purpose is the presentation of a deformed destructive being (DDB), commonly known as a monster.
By this standard, Super 8 is not a horror movie. (SPOILERS ahead.) Its primary purpose is to present a world in which Earth people (mainly some kids making a Super 8 movie) interact with an interstellar visitor. The visitor is deformed and destructive, but he is ultimately peaceful in his intent; he just wants to go home. By the end, humans and alien understand each other and make friends. All this makes it a science fiction movie, not a horror movie.
This is too bad. It is a missed opportunity. Super 8 is not terrible as it stands. The first half of the movie is fascinating to anyone (like me) who actually made Super 8 movies as a kid. It is accurate down to depicting the minimum three days required for developing a roll of Super 8 film. It engagingly captures the scrounging for locations that kids with zero budget have to do in lieu of building sets or flying to faraway shoots. But the second half of the movie, once the monster is loose, is weaker, although still enjoyable in a conventional Spielbergian flight-of-fancy sort of way.
The thing is: these kids with the Super 8 camera were making a horror movie. To be specific, they were making a Romeroesque zombie movie. Super 8 would have been much more fun, and more coherent, if they had been given what they wanted, and they had walked into an actual horror movie. The monster could have been the beast that was presented, or something worse. But it would have been more fully destructive, and it would not have been sent home peacefully in its spaceship at the end. It would have been destroyed or would have lived on in a chilling frisson.
Incidentally, the kids should have kept their Super 8 camera rolling all this time, and worked footage of the real monster into their finished film, presented over the end credits. As it stands in Super 8, the Super 8 motif is forgotten once the action heats up, and only resurrected in the end credit roll.
Whether Super 8 really would have been better as a horror film is hypothetical, of course. Unless someone hauls out their old Super 8 camera and shoots this alternate version, there is no way to compare the two. But something is lacking in Super 8, and it well may be horror.
Deformed and Destructive Beings: The Purpose of Horror Films