One common tack in urban fantasy is to assume All Myths Are True; or at least that all folkloric critters are real and may attack the locals, show up in your occult investigator’s office, or prank-order pizzas to bury his driveway. This has its ups and downs… and no, I’m not talking about flying monsters.
The upside is, just like people in a city might come from anywhere, so might the monsters. If you can meet a Japanese businessman on a random street corner, why not an oni? Or a kitsune? Or a sleeve-haunting sparrow?
(No, really. In some areas of Japan they’re supposed to warn you when their equivalent to a Black Dog is stalking you. In other places… not so much, you’re on your own.)
Urban fantasy often goes for a worldwide, cosmopolitan feel. If that’s what you’re after, sure, throw in creatures from anywhere. The world is your oyster!
But what if you don’t want to evoke a generic Metropolis in your setting? What if – urban fantasy or not – you want to portray a very specific setting? A place that’s a character in its own right, with moods and weather from that front off the Great Plains, or this storm blown in off the Atlantic. What should your monsters be like then?
If that’s your aim, I’d advise you to pick up two things. 1) Folklore of the closest place to your setting possible, and 2) any Monster Manual-type book that has environment or habitat as one of the creature stats on its listings. Then pick monsters that fit your setting.
There are reasons the Gargoyles cartoon worked so well. One of them is that New York City is already a city of gargoyles (and grotesques). They crouch atop buildings. They cast ominous shadows in the lightning. They spit water in places you’d never expect. It’s not that much of a leap to imagine them gliding on the winds whipping between the skyscrapers, clawing their way up stone and brick, and watching over the city like Batman with fangs and wings.
You wouldn’t put a cactus cat in a New Orleans swamp; the poor thing would drown. You wouldn’t have a kelpie galloping through the streets of Vegas – not unless it’s learned a knack for knocking open fire hydrants to keep from dehydrating. (Now there’s an interesting insurance headache.) Give your monsters reasons to be where they are. Even if that reason is some nitwit shipped a crate to sunny Miami that was supposed to freeze in Antarctica instead.
Local folklore gives local flavor. Consider how much mileage you can get out of the monsters already in your neck of the woods. Then consider if you really want a foreign monster, or if you can tweak a local to fit your City of Adventure even better. If your city has a lot of Irish inhabitants, what’s more likely to be haunting the docks – same-bito or cranky, soul-stealing merrows?
Of course, maybe you want a supernatural turf war. In which case they’re both there, and the sailors are terrified….