There are three basic types of horror film: the supernatural, the scientific, and the naturalistic.
The first type (which we most often think of as the true Horror movie) is where we come across supernatural monsters such as vampires, werewolves, demons, ghosts and zombies. These are creatures of legend and come from thousands of years of human psychic response to the mysteries of the earth.
The scientific horror film is the type where we find the mad scientist(s) creating (or discovering) something outside of nature; something that will be lost from human control and run amok before being overpowered (usually) by the military, and the world is restored to peace and harmony. The monsters featured here are often new creations, things we've never imagined, never mind seen. Sometimes they are not made or found by Man – they exist just ouside of human awareness until they feel it necessary to appear and endanger the human species.
The third batch of horrors are the naturalistic monsters which our daily news is often reporting. These are the serial killers and the psycho murderers, the madmen and the crazies. Sometimes these creatures are created for the screen from the very real life atrocities that scream from the headlines. Or, indeed, are perhaps covered up by the media for fear of copycat crimes. Other times the monster is an animal, such as a bear or a reptile (usually grown to above-average size), or even a group of beasts (ants, bees, spiders, dogs).
Now, none of this is simple and strict categorisation. Some monsters may be difficult to slot into a certain genre. For example, the giant bear that runs amok in Grizzly could be hungry due to a lack of his usual foresty foodstuffs (creating a naturalistic horror), or he could be a scientifically altered, perhaps by manmade chemicals in the water supply, to reach a monstrous size and therefore unable to not eat humans to supplement his diet. And the terrors o