A monster is any creature that does not exist in the real world. Dragons, goblins, and various aliens from the movies are examples of monsters.
Humans seem to have a fascination with monsters, they have been filling their folklore, fiction, and popular culture with creepy crawly things since the beginning of time. Perhaps it is the fear of the unknown? Or more likely monsters are embodiments of the faceless fears of the dark, loneliness, and death that have plagued almost everyone since time began.
Every culture has its own monsters, even the modern day western world (although now they are called "aliens"). Many types of monsters are oddly widespread. The vampire, lycanthrope, and dragon in particular have appeared independently in many far flung cultures (which leads one to wonder, did they really exist at one time?)
Sea monsters are especially prevalent in any culture that lives near the water. Over hundreds of years we have found that some of those "monsters" were indeed real (Giant Squid for example). Lake monsters resembling the plesiosaur have been reported for thousands of years, but scientists have yet to discover a body.
When designing a monster for use in a movie, game, or fiction, you must take several things into consideration.
Monsters have to eat
This one simple fact is overlooked by almost everyone. A dragon, for example, will be the largest predator around for hundreds of miles. It will most likely live in a cave, and the entire area for at least 50 miles will be plagued with the constant slaughter of herd animals (needed to support the body mass of the beast).
How about the case where the space marines go to the abandoned station/outpost to find it has been overrun with carnivorous beasts. What have they been eating between the time they overran the area 3 years ago, and today when they start eating the crew of the newly arrived ship.
Mammals and insects especially need huge amounts of food to survive. You can't just have a planet populated by billions of giant insects, with no other species. It doesn't work.
So always design a ready food source for your monsters. Humans are a poor choice, as they take too long to grow to maturity. Herd animals, plants, and insects are a much better choice. These are easy to work into your story or adventure. You just have to think about it.
Monsters living in peace! Real or Malarkey?
Fantasy RPG designers have a real problem with this. They will have one small dungeon, with 15 different species of intelligent monsters living in it, seeming oblivious to each other's existence. It doesn't work that way either. The most powerful species will subjugate any and all other species in the complex. They do not just live in harmony, that is not how evil works. To properly account for this you should move the bulk of the valuables in the complex over to the most powerful monsters, while making sure they have the best rooms, and so on. Don't forget that all these monsters will have to eat (making the weakest species do some sort of agriculture works nicely).
What kind of power is that?
People like to make their monsters fantastic, with abilities beyond what earthly creatures can accomplish. This is fine, but be realistic. No monster will have ever developed nested heads that come out of the largest one. Any small creature of immense power (like a beholder), will need to have some inter-racial squabble to keep them from taking over the whole planet. Resist the temptation to create cool looking monsters that would have never came about in any sort of natural environment. Avoid abilities that could simply never exist (it is conceivable that a creature could develop a flame breath, it is not conceivable that it would develop the ability to transform into a mist).
The best way to create a realistic monster is to find the equivalent creature on earth, and use that to fill in the gaps. Look at wolves for inspiration on how medium sized carnivores actually live. Lions, elephants, monkeys are all good inspiration too. If your creature is intelligent, and has a way to build tools, then there is only one species to look at for inspiration, man. Your gopher-men should have as much detail as real humans, they should have goals, religions, taboos, and customs. They should manipulate the environment around them, in the same manner that humans do.
Here are a few monster nodes to get you started (just send me a /msg to have your node added).
- creeping mold
- blue mold
- gray mold
- Brain Mole
- broken one
- deep dragon
- violet fungus
- gas spore
- Fog Giant
- bag of devouring
- Living Wall