"S" is for "Slimebiter"

Slimebiter

No. Enc.:  1d4 (2d4)
Alignment:  Neutral
Movement:  45' (15')
     —Swim:  150' (50')
Armor Class:  5
Hit Dice:  8
Attacks:  3 (2 claws, 1 bite)
Damage:  1d6 / 1d6 / 2d8+2
Save:  4
Morale:  11
Hoard Class:  VI
XP:  4,060 (or 4,560)

Urban legends abound in the Mutant Future about giant, mutated animals—rats, snakes, alligators, and even sharks—infesting urban sewers, but there's no way such creatures could possibly exist...

...because the seals would get them.  It would be crazy to think otherwise.

Slimebiters (also known as "sewer seals" or "sludgeseals") are 12'-15', 1.5-2 ton pinnipeds that inhabit the most contaminated of waters:  derelict sewers, toxic ponds, reactor retaining pools, cesspits, and the like.  They lurk beneath the mucky surface waiting for prey, and then explode forth with such speed and ferocity that they Surprise on a roll of 1-4 on 1d6 and gain a +2 Initiative bonus.

Slimebiters' mouths swarm with disease, and their bites transmit either Flesh Eating Bacteria (50%) [see p. 48 of the Mutant Future Core Rules] or Tetanus (50%) [p. 49] on a failed Saving Throw Versus Poison.  They, in turn, are immune to the effects of all known illnesses, toxins, and radiation.

The creatures are one of the few known predators of glowbras, quicksilverfish, and repulslugs.

There is a 10% chance that any given slimebiter possesses the Density Alteration mutation.  These size-shifting monstrosities are powerful enough to burst through the pavement covering their sewer lairs...and rampage throughout the city above, devouring all in their paths.

Mutations:  Enhanced Vision (Night Vision), Reflective Epidermis (Radiation), Toxic Weapon ("Bacterial Bite")