There's a really nice one in Pathfinder #7: Edge of Anarchy. I know that this helps you none at all. But in summary:
rats, in swarm and dire versions
wererats, perhaps accompanied by the former
ghouls and ghasts
otyughs, of course (perhaps struggling for sewer scavenger supremacy with carrion crawlers)
alligators, if it's in a tropical region or you're feeling facetious
ordinary humans, either beggars or criminals on the lam
giant spiders and centipedes
Unique to Golarion (the setting of Pathfinder) in its sewer association, but something I really like: derro. In the setting, derro are the little blue men (think aliens, not Smurfs). They abduct people at random, drag them underground, paralyze them, conduct horrible and inconceivable medical experiments, then return them hours or days later.
Demiurge out.