Urban Nightmares
Joshepha Sherman and Keith R.A. DeCandido, Editors
Four of the twenty-five pieces in Urban Nightmares, a collection of short stories based on urban legends, feature sewer saurians:

Gator, by Robert J. Sawyer
Alligator? You wish.

The Release, by S.M. Stirling and Jan Stirling
Evil animal rights activists get theirs.

What a Croc!, by Bill Crider
Captain Hook in the sewers.

My Naggilator, by Lois Tilton
Little girl outgrows Barney. Is the replacement even worse? Is such a thing possible?