What should be considered an urban monster?
The only D&D creature that could fit in that category, in my opinion, could be the wererat.
Could be, the gremlins, considered an urban monster?
I can think of a few: The splinterwaif,, from 3.5e Monster Manual III:
The splinterwaif is an evil variant of the dryad that inhabits the wood used in buildings and preys on city inhabitants (particularly children)
The 3.5e book Cityscape also has a handful of creatures, such as the Siege Golem:
Siege golems are most frequently used as one element of a larger military force. They might be deployed in groups, and are often accompanied by other creatures that defend them from attack, allowing them to focus on a wall with the catapult attached to its back.
the Cesspit Ooze:
The cesspit ooze is a product of the urban environment, a horrific predator born of the distinctive mixture of pain and despair found in the poorest quarters of the city combined with the ambient arcane energies that emanate from the most magic-intensive regions. Their foul, brown-green sludgy bodies often contain garbage, rusted tools, old rags, bits of bone and rotted animals, and raw sewage.
the Ripper:
The ripper is a vaguely humanoid aberration that lurks in the darkest, poorest urban areas. There it preys on its favorite meal: the flesh of sentient creatures.
the Sepulchral Thief:
A sepulchral thief is an undead criminal, a thief, burglar, or bandit. Unlike other undead whom it resembles, such as the lich, the sepulchral thief does not deliberately seek out an undead existence. Rather, like some ghosts or ghouls, it rises spontaneously on rare occasions when the proper circumstances align.
Sepulchral thieves are shrouded in a mind- and senses-clouding aura of negative energy.
And the Zeitgeist:
The zeitgeist is a strange, phantasmal fey that appears to be generated by the lives and emotions of those living in an urban center. It is capable of assuming multiple forms; each is drawn from the building blocks of the city and culture.
A zeitgeist's three forms are:
1. Mob manifestation: it takes on the form of a huge mob of humans.
2. Smoke manifestation: it takes on the form of an enormous cloud of semi-solid rolling smoke
3. Stone manifestation: it takes on the form of a huge humanoid figure, composed of stones, detritus, and even whole buildings.
