Speculative fiction is a stretching of the world into new shapes, an expansion of ideas to their logical (or illogical) conclusions. One of the many ways in which writers test those limits is by the inclusion of creatures that don’t exist in our world. There are good reasons to consider writing about monsters in a story, and there are some excellent reasons not to include them.
If you just want something for your hero(es) to fight, you should consider possibly delving into RPGs. Random encounters aren’t good storytelling. An encounter should advance plot or character development somehow, not just give the hero(es) XP. But if you want to make your world a more dangerous place, give a sense that humanity is not in control, then the addition of monstrous creatures is justifiable. In the real world, there is no longer any place on Earth that we can’t go because of animal life. Thanks to guns, chemicals, and frankly, bulldozers, humans have taken over. Putting a reader into a world where humanity isn’t the top of the food chain (or at least is less secure there), can add flavor to a story.
Here are three types to consider:
The night has always held an air of menace. Things come out to hunt when the sun has gone, and humans are at a disadvantage defending themselves because they cannot see what’s coming. But sometimes wolves and jungle cats just aren’t enough to chill the blood. You need something couched in superstition, mystery, and misinformation.
A boogieman can be anything that people fear in the night, which they don’t have personal experience with. Most often, there will be stories passed from one generation to the next. Finding someone who has firsthand experience with the monster will be next to impossible; often there will be a segment of the population who doesn’t believe in the creatures. When writing this sort of monster, you can go either way with it being real or not. Most time it can fulfill the story needs either way.
The Misunderstood Creature
Similar to the boogieman, this creature is the subject of rumor and misinformation. Unlike the boogieman, people have personal experience with it that have given them an incorrect assumption about the creature. They may misunderstand its motives, its place in the food chain, or its intentions toward them. Often, such a creature has at least a rudimentary intelligence, if not fully human-like intellect.
This sort of creature can be used as a mirror for the fears of the people who create the stories. They project what they fear the most onto the creature, and blame it for unexplained tragedies.
Note that no matter how misunderstood the creature actually is, that won’t matter if you never offer readers a perspective that shows the “monster’s” true nature.
Not every problem can be solved with brute force, and this is a creature that resists all attempts at being solved in that manner. Swords, arrows, lasers, and bombs, it shrugs them all off. Only by understanding the nature of the creature can it be overcome. Often, this will result in the creature being placated, trapped, or driven off, not outright destroyed, but there can also be cases that once the monster’s weakness is known, then it becomes killable.
This is a tricky one to use, because often the answer may come across as deus ex machina, but done well it can change your focus from a hack-and-slash heroic fantasy into something based more around guile and cleverness.