Monsters in General
I would like it to be clear to start with that I'm using the word "Monsters" currently as a blanket term, under which I am shoveling everything; from Godzilla to Gremlins, Zombies to Werewolves, and Frankenstein to Hannibal Lector. I am talking about all the evil creatures and villains that make horror movies horrifying. Some specifics types I'll go into more detail further on. For now, this is where I'm starting though.
The Horror genre in general, has lost a lot of it's main monsters. Some have passed away from age, as the fears they engendered have given way to a better understanding of the way the world works, or they were passed up by things even more frightening. The age of the giants is long past as well as the fears of the atomic age. We are no longer frightened by the lack of understanding we have of nuclear weapons, we know we are not going to be mistakenly creating colonies of giant ants or turning iguana's into hundred-meter tall monsters.
Another trend which seems to be stealing away a lot of the power of Horror's most basic tool is the trend toward the Young Adult demographic. This group isn't interested in reading or seeing things that scare them. Anyone who has talked to a teenager for any length of time is well aware that their whole lives revolve around the drama of relationships, their own, their friends and those of the people they like or are interested in. Thus, a lot of the power our monsters had is getting toned down for this demographic or in some cases, even taken away entirely and then romanticized! One of the bigger problems with this is that as people move out of the YA category, they have already come to expect that of the monsters they may see in later books or movies and thus they dislike it when the "true" monsters come out.
Vampires are one of the oldest monster tropes we have in the Horror genre. They have also grown to be among the weakest ones still shuffling along. While they survive in adaptations such as 30 Days of Night, Blade, and Underworld, the damage they have suffered in the past several years has been truly appalling. Gone are the days when a single one of these lords of the undead could send shivers down your spine and cause you to check the doors and windows before bed. Their strength has been weakened by such creations as the Twilight series, Being Human, and The Vampire Diaries; now they only cause fear by running in packs and acting like savage predators. It is telling though, that as a society we no longer fear seduction by these creatures, which was always an allusion to the sexual repression of the victorian era. So it has to be said, as much as we have outgrown this classic monster, it is a testament to it's power and adaptability that it remains such a common trope, whether or not it is portrayed as the monstrous villain or the courageous hero.
Werewolves are another Horror classic, and they have remained largely unchanged since they first appeared. The werewolf was always used to represent the duality of the human condition, the difference between the intellectual and the beast. They aren't as common on their own and usually when they do show up, as in the Underworld movies, they are second string to vampires. Werewolves, however, remain one of the most consistent monster tropes that we still have. The human 'beast' who has no control over his transformation and joins with others of his kind to roam in packs for strength and protection. The point being, human beings are pack animals and as long as we strive to maintain our intelligence, there will always be the struggle between the brain and the animal instincts inside us.
The zombie didn't become a horror trope in and of itself until the 1968 film The Night of the Living Dead, though several books and films did mention them in previous years, such as Richard Matheson's 1954 novel I am Legend. The idea of the zombie, a person who is dead moving around actively looking for live flesh to feed upon, hits upon several things that scare us; such as a loss of identity and cannibalism. Unfortunately, the zombie trope has been somewhat overdone in the past decade or so and has lost a lot it's ability to frighten us. There are many ideas as to why that is. Commercialization for a start, but I think it's more to do with the fact that you can have too much of a good thing. Zombies have been featured so heavily over the past several years that it's almost become a common part of the normal world. Movies like Shaun of the Dead and Warm Bodies make light of the zombie as a monster and even in shows that take them seriously as in The Walking Dead make them almost secondary to the drama of the survivors. Zombies will continue to stick around, but I expect them to further degrade as the years continue, not because we no longer fear what they represent, but because we have more or less given in to it.
Ghosts and Demons
It's rather sad to me that these are almost becoming mixed. Interest in these things has spiked, due in large part to the show Ghost Hunters and movies such as Paranormal Activity. These tropes as monsters offer a look into some of the greatest fears we have; the unknown and what happens to us after we die. The idea that we can only see what they want us to see, whether it's a door moving or pans rattling feeds a sense of paranoia that they can be all around us at any given time and at their will could grab us and throw us against the ceiling or drag us down the hall. Unseen spirits are one of the most enduring tropes in the horror genre and despite the passage of time have lost none of their ability to frighten. In some ways their ability has been strengthened by the jump in interest through the tv series and movies I've already mentioned, which have allowed everyone to see what these things could be capable of. It gives me hope that as long as the question of what these things really are remain unanswered, these will remain capable sources of frights, for books and movies.
We have come a long way from fearing the silver space suits of the 1950's, and despite a seeming wane of interest these ideas continue to cause fear. It is a fear that has matured since it was first used, and no longer do we fear simply seeing the ships in the air as we did during the days of the cold war. Instead, we discovered what to truly fear, which is what they can do and why they're here. This is evident in movies such as The Fourth Kind and the older Fire in the Sky. As what it is about them that scares us has solidified, so has the level of terror they can bring us to. They have become the proverbial 'monster in the closet', waiting for us to go to sleep and sneak into our bedrooms. They have come to embody our fear of the dark and of what is out in the vast emptiness of space. They are currently taking a break, stepping aside for the uprising of the demons and spirits which are occupying the main stage of the horror genre currently, but they remain a powerful source of fear and frights and will likely see another resurgence in the years to come.
Those are the main monster tropes available in the horror genre currently. There are others of course, there has been a slight resurgence of the classic fairy tales, updated for the modern age and returning to their darker roots in the shows Grimm and Once Upon A Time. Giant monsters still pop up from time to time like the forthcoming movie Pacific Rim, as well as the monsters created by science be it Jurassic Park, or Rise of the Planet of the Apes. There are also the human monsters, reveling in the horrific acts that people can inflict upon each other. Rarely, a brand new abomination will rear it's head. The point of all these monsters and beasts though is mere allegory. No matter what repulsive forms they take, they are manifestations of the things we fear the most. Not everyone is scared of these things equally and some might fear other things entirely, but that is the truth of what these creatures are, and until these fears are conquered by the collective human society, they will continue to roam, terrorize and kill in books, movies and television. And I for one, don't think that's a bad thing.