1. It was sealed away, but now it’s been released
This is perhaps the most common type of monster. These are the ancient creatures that have long been relegated to myths and legends, or the creatures we never even knew about.
Some, like King Kong, are trapped in an isolated part of the world. Others, like Lovecraft’s Cthulhu, may lie deep underground, or in the farthest reaches of space. In each case the monsters are separated by some kind of barrier that keeps them from intruding on the human world. It may be a physical barrier like the ocean that surround’s Kong’s island, or a barrier of distance or time. Both Lovecraft’s Cthulhu and Tolkien’s Nazgul wait for the proper time in which to return.
2. It was hidden but now it’s been revealed
Sometimes the monsters have always “been here”, we just didn’t know about them. Vampires are the most common example. As a monster that has the intellect and appearance of a human, it’s easy to imagine such a creature blending in with society. And since they’re hiding, they can’t afford to let word get out. Anyone who knows their secret must be silenced.
3. It never existed until now
Throughout history humans have often tried to improve upon nature’s design, trying to enhance the minds, the body, and the lifespan of a human being. Whether through magic or science, fiction is full of stories where someone creates a chimera, cyborg, golem, or zombie.
4. It is openly active, pursuing its agenda without any effort to conceal itself
Sometimes stories start with monsters already at large. J.K. Rowling’s Harry Potter and David Edding’s Elenium take place in a world where trolls, ogres, and giants roam the world at large. Of course as a natural part of the diegetic world, some might say that these are simply creatures, and not monsters at all, but since they are not part of our world, and represent a threat humans cannot easily overcome, I would still consider them monsters.
102-10 Lamentable Flaws