"M" is for "Maulrat"

Maulrat

No. Enc.:  3d6 (3d12)
Alignment:  Neutral
Movement:  150' (50')
  —Burrow:  15' (5')
Armor Class:  4
Hit Dice:  1d4 hp
Attacks:  3 (2 claws, 1 bite)
Damage:  1d3 / 1d3 / 1d6
Save:  L1
Morale:  10
Hoard Class:  VI, VIII
XP:  9

Maulrats are hideous, 1' tall, bipedal vermin with bony carapaces, segmented tails, and multiple never-blinking eyes (which prevent Surprise). Their frenzied hopping gaits would almost be comical if their pincers and fangs didn't guarantee mutilation and evisceration.  The creatures can eventually gnaw/tunnel through any known substance.

Labwraths often breed maulrats to both patrol their lairs and serve as living "disposals" for their failed experiments.

There are rumors of unique Pure Human and Mutant Human children (usually societal outcasts...vengeful societal outcasts) that can somehow command maulrats to do their bidding....

Mutations:  Extra Parts (Eyes), Increased Sense (Smell), Natural Armor (Extreme)