I’m sure this is a problem only for certain genres:  Your heroes (or victims) are traveling through their adventure and they’re attacked by a tentacled horror.  Wait!  Maybe it’s a flying grizzly bear.  I could go old-school with a chimera.  People might not know what that is, so maybe a dragon.  That’s been done.  I got it!  A Penguin Dragon!  Eh, maybe I’ll just uses goblins again.

A key component of adventures is the ‘monster encounter’.  These could be in the wild, summoned creatures, or guarding the final goal.  Most stories that take place in another world have some type of fictional creature.  These are the beasts that fill your characters with dread or annoyance.  They also help the reader believe that this is a world with wonders and the author put some work into it.  If you’re only using dragons, goblins, and the standards then you might lose some interest.  On the other hand, you have to be careful about going too far with your original creatures.  Preferably, things with several abilities should be saved for important scenes.  A flying , fire-breathing, life-absorbing, blade-wielding squid that can pass through solid objects could be a clear sign that you went too far for a simple encounter.  It could also be a sign that you should lay off the ‘shroom and moonshine breakfast.

Is there a perfect way to do this?  No, but there are various paths you can take.  I’m going to say now that you don’t have to follow a single path.  Jump around then and jumble stuff up for your world.  Let’s see what we have.

Traditional Monsters

There is a vast array of monsters found in mythology and fiction. You have the standards of dragons, the unique of Vitterfolk Trolls, and just a mountain of monsters.  There are bestiaries that you can buy or just wander Wikipedia looking for them.  This is a relatively easy method if you’re good at researching and have an idea of what kind of monster you want.  An initial encounter for young heroes shouldn’t be the three-headed dog Cerberus and experienced heroes shouldn’t be wasting their time with kobolds.  A great example of using obscure creatures in stories is done by Robert Davis, who wrote a short story involving the Rakshasa.  This is a Hindu demon that not many people know about.

Natural Frankenstein

A common method for unique monsters is to take a base animal and add stuff onto it.  For example, I created a creature called the Eusu.  This is a large crocodile-like animal that can split its mouth into four jaws for a bite that comes from four sides.  This type of mouth is fairly common.  I also gave it the ability to detach its tail and regrow it within minutes.  So, you can see that I took a natural animal and added strange abilities to it.  This isn’t as hard as it sounds, but you do have to take your time with it.  I’ve found that this method also works best when you need to make a monster in the spur of the moment.  The heroes are heading into a dungeon and you forgot to plan a door guardian?  Grab a kangaroo, put an axe on its tail, horns on the head, acid breath, and a mouth in the stomach.  Now you have your dungeon guardian.

Everything is Bigger

This is the lazy man version of monsters.  Basically, you take regular animals and make them bigger.  That’s it.  Not original, but anyone that does this exclusively probably isn’t aiming for originality in the first place.  I hope they have an explanation for it.

Demons and Undead

These get their own small shout-out because they differ from standard monsters.  Undead are simply reanimated versions of other creatures, so you can go anywhere with this.  Most people stick with humanoid, but imagine a zombie centaur or a skeletal troll.  As for the demons, there is a rich history you can pull for them.  Every culture and mythology has some type of demon.  They’re typically easy to make too because you aim for evil, gross, and/or vicious.

Platypus

There are some people that can easily create a monster out of nothing.  An abyssal black hole with clawed limbs or a moss-covered, six-legged beast that rolls in a ball.  Some aspect of another creature might be in here, but there’s no clear core to these things.  This takes a lot of imagination and creativity.  Unlike the natural base, you need to take time to design something like this.  Left unchecked or in spur of the moment, a creature like this can grow into something unbeatable.  You can find yourself adding powers and resistances to prolong the battle until the heroes can’t win.  Be careful with this one.

Again, you can use a mix of all this to flush out your world.  Here are some final guidelines:

1. Be consistent with what a creature can do, especially if you reuse it.  Trolls that are weak against fire can’t be immune later on without an explanation.
2. Every monster has a weakness.  These things are designed to be slayed unless you want the ‘twist’ ending of everyone dies.
3. For common monsters, it helps to create a sense of what it’s purpose in the environment is.  Make your monsters part of the natural order instead of some odd beast that appears out of place.  Apex predators are the common ways to go.
4. Don’t introduce every new monster with it eating a lion, a bear, a shark, or some other kind of top predator.  That gets old.
5. Have fun.