When I started writing the fourth Void City novel (currently titled Hunted) I had a problem. Wererats were nearly what I used to fix it, but that would have totally wrong.
See if you can follow my thought process:
In any ecosystem, when one natural predator dies out or is diminished, the ecological niche they occupy is taken by another predator. So... with the reduction in the total number of vampires in Void City after the events in Crossed (Void City, book 3), it seemed only natural, that I bring in wererats. Until I started writing it and the wererats didn't show up.
I didn't know why they wouldn't show up. They simply didn't. I had a problem and my conscious brain didn't know it, but my subconscious totally thought I was being a moron. My subconscious was all about the chupacabra. I caught myself googling them and complaining about how much cooler they could be... and then going back to trying to shoehorn wererats in book 4.
When I finally tried putting them in, it was obvious they'd been the right creature all along. Vampires drink blood. Chupacabras drink blood. Wererats don't drink blood... Of course then I had to think of ways to make chupacabras cooler. After all, these things were going to go toe to toe with Greta (my female vampire uber killer and daughter of the main character). That's when I stumbled on other funky ideas like mobbing behavior and how, in the news, whenever they kill a "chupacabra" it's always revealed to have been nothing more than a parasite infected coyote or similarly mange-ridden animal.
After that, it got even easier. A pack of chupacabra aren't a pack, they're a plague (like rats) and they don't have Alphas, they have Prime Contagions and... what if they can infect humans and turn them into chupacabras with the quills that run down their backs and tails? And... And.. And...
In other words, a floodgate of data opened up about why these monsters are actually quite scary and so, when Greta meets them (instead of the wererats she was originally going to uncover) it makes more sense. Here's an unedited excerpt from Greta's POV:
Seven curious creatures sniffed around the edge of the hotel yipping to each other in a series of hisses and barks, that might have been a language. Cracked grey skin hung from their muscular forms, with patches of fur turned greenish-gray by the presence of what smelled like moss or lichen. A ridge of tuberous spines, larger and thicker than, but similar in principle to, a porcupine's ran down their backs growing longer and sharper at the end of their kangaroo-like tails. Broad boar-like muzzles ruined the canine-cast of their muzzles and dark black forked tongues that tasted the air. Claws tipped the fingers of their very prehensile looking hands, but even bigger talons curved raptor-like at the ends of each massively splayed-digited hind paw.
Their scent, equal parts: sulfur, iguana, and dog... with a hint of lime, rolled about my nostrils, catalogued, not to be forgotten. The stink of human fear coated the air, too, but not in equal measure.
"Look how they herd that hooker and her john," I elbowed Evelyn. "They aren't bad. Watch how the big one is deliberately leaving an opening for one of the two to bolt."
"That's on purpose?"
"Sure." I stood. "They could tear the two of them to pieces, but they're careful. I think I like."
The hooker, a bedraggled blonde in hot pants opened her mouth to scream, but when she did, the creatures hissed, tongues vibrating and extended. Her face twisted in confusion and she tried to scream louder, but nothing made it to my ears except the hissing. "Noise cancellation." I stepped up onto the steel rail and leaned out for a better vantage. "How cool is that? I didn't know chupacabra could do that?"
Have you ever run into a point like that in your writing (or reading) where the scenes could be fixed by just putting the right monsters in it? What are you favorite underused monsters?
(Note: I should point of that the not only is that excerpt a clip from an as yet unpublished novel and is copyright 2011 Jeremy F. Lewis, but that there are other FREE samples of my fiction over PocketAfterDark.com .... a free short story about Vampires and Chocolate and another vignette of Eric and Greta at the beach) While you're over there, you could even vote for Void City as your favorite Urban Fantasy city.