New Monster: The Piscapede

Use the stats for a shark from the 1e monster manual (or wherever).

These creatures resemble enormous eels with hundreds of centipede-like legs. They dwell in pools of stagnant water that it makes opaque via chemical secretions. The resulting water color is often quite vivid and unnatural. Two adult Piscapedes will dwell in a pond at least 15’ x 15’ with 2d4 legless young. The amphibious adults will crawl out of the water and attempt to bite an intruder. A successful bite attack forces the PC to roll a saving throw (vs. Poison); failure causes the PC to lose the capacity to breathe air and grow gills. The PC must then submerge his gills in water within (1d4+CON modifier) rounds or faint from lack of air and begin losing HP. Once underwater, the opacity of the fluid grants the young near-invisibility (-3 to hit). The venom will lose its effect in some 20 minutes, and a clever adventurer can extract the poison sacs of the adults to create a potion of water breathing. 

These creatures are most often found underground, and can be considered dangerous pests in some extensive sewer systems.