This is for all the fantasy fans out there but I suspect there may be a bit of character building information in here also. I say, I suspect, because I haven't written it yet and I just don't know.
So, do you prefer building your own creatures or going with the tried and true? Little of both maybe? That's me, a little of both. I use dragons and wyverns because they are just so cool. Bad-ass, usually sentient, able to attack from the air or ground, how can you top a dragon? They are almost too cool to be really scary. Fascination gets in the way of the fear. Also, how many times can your guys battle a dragon and win? Once? Okay, maybe, but more than that and it's not believable. We need other, less formidable foes. But, there is one quality these lessers must have.
I need stuff that scares the hell out of my good guys.
It's for their own good. How could they think of themselves as courageous if they never faced something that made them want to run home screaming and waving their hands? And I need them to know they are courageous or they will not do courageous things. (I used courageous three times in that paragraph to illustrate the usefulness of repeating a word deliberately. For impact or some such thing they told me. Just don't spread them out all over the page. They need to be close together or it looks like you don't have a good vocabulary. That's the theory anyway.)
Back to building scary creatures. How do you make them scary? Their method of killing is one component. What's more frightening than being eaten alive? Nothing. So I built a creature to do just that. I started with an octopus. They have a nasty beak at the center of their arms, perfect for biting off bits. Ouch! Then, I made the body the size of a sheep. Uh-oh. Then I gave it claws like a sloth's and put it up in the trees where it hangs upside down. As it turns out, the supot is a mindless solitary ambush hunter that drops on its victim from above, wraps it up like eight constrictors, bites repeatedly to get blood flowing then presses its fleshy outer lips to the wound and sucks the prey dry. You'll want to be very careful wandering the woods in supot country.
What else is scary? Big, big is scary. In this case, I borrowed a creature from my friend, Marne. She invented a disgusting, smelly, gargantuan, man-like thing called a golgea. It's similar to the cave-troll in 'Lord of the Rings'. Great for cooperative heroics on the part of my protags. However, mine are dwarf golgea, only seven feet tall.
Sneaky and stealthy is a worrisome thing in an opponent. Nepdak change the color and texture of their skin to match their surroundings. And, they attack in groups. Like camouflaged werewolves in a pack.
Okay, there are three creatures I built for my current project. There are more but you get the point; pick something scary and make a creature to fit. What's scary? Getting strangled. What strangles? A snake. Can't be just a snake. How about a two hundred pound carnivorous squirrel with a long muscular prehensile tail. It clings to the trunk of a tree or bough and whips its tail down around the neck of unsuspecting prey then hauls them up the tree, hanging the unfortunate animal...or person.
You can do this with your human characters too. What would be frightening in a person? A fascination with edged weapons. Okay, but it can't just be a guy with a knife collection. How about a skinny pencil necked guy with an overbearing mother and a collection of vintage surgical tools?
Got creatures? Tell me about them.