Over time, metropolises like the City extend urban sprawl into areas which were formerly wilderness--wilderness haunted by monsters. Some monsters are pushed further out into the wilds, but others adapt to the new environments in which they find themselves. Then their are other creatures who have always been associated with man and his habitations in a variety of ways, from freeloader to predator. Here's a sampling of creatures one might encounter in the City and perhaps other urban areas:
Oozes/Slimes/Molds: These organisms can be found in underground areas like sewers and subway tunnels which they colonized from deeper strata of ruins, or where they were dumped by irresponsible alchemical concerns. These organisms have been known to spread up into basements, or even through the walls of decaying buildings. Such incursions often occasion a call to the Municipal Department of Animal and Pest Control.
Ghouls of the Strange New World are not actually undead, but have a taste for brains which make them dangerous, particularly if handled less than courteously. Vampires--blood junkies--tend to slide over their unlife from more respectable areas to slums where their victims are less likely to be missed.
Devils: The Hell Syndicate prefers to use human agents, keeping its infernal troops in reserve, but there are a few exceptions. Succubi and incubi are heavily involved in higher-priced prostitution rings. Hit-Fiends, mostly disguised (barely) as mundane muscle, back up particularly high-placed human gangsters, and are sent rub out particularly annoying adventurer-types.
4 hours ago