"M" is for "Matterpus"

Matterpus

No. Enc.:  1d2 (1d4)
Alignment:  Neutral
Movement:  60' (20')
      —Swim:  60' (20')
Armor Class:  4
Hit Dice:  5+2
Attacks:  3 (2 claws, 1 bite)
Damage:  1d6 / 1d6 / 1d4
Save:  L5
Morale:  11
Hoard Class:  X, XIII
XP:  860

A matterpus is a 3'-4' long, duckbilled creature that dwells near polluted bodies of water.  Its oily, luminous fur coruscates with strange energies, as do the two fleshy growths sprouting from its head.

Surly beasts, matterpodes are highly aggressive and territorial.  Not only do their claws inject a Class 8 venom, but their antennae project beams of atomic force that deal Class 5 radiation damage.  Furthermore, anyone struck by such beams must make a successful Saving Throw Versus Energy or suffer the following additional effects (determined by rolling 1d8):

1) Target (but not possessions) is transformed into unmoving stone for 1d4 days.
2) Target's possessions (weapons, clothing, armor, artifacts, etc) become vaporous and dissipate into nothingness.
3) Target's armor transmogrifies into a softer material, acquiring a +3 Armor Class penalty.
4) Target's ammunition is rendered inert.  Bullets/shells on person decrease in number by 2d8, and batteries/power cells lose 2d8 charges.
5) Target acquires the Bizarre Appearance Physical Mutation Drawback (specifics determined by the Mutant Lord)
6) Target's blood turns to acid, doing 10d10 damage.
7) Target's teeth, skeleton, and nails transmute into metal, gaining +3d6 damage to unarmed melee attacks and +2d6 Hit Points
8) Target's weapons and equipment rendered into brittle—but valuable—gemstone crystals worth 1d20x1000 GP.

Matterpus lairs are always littered with precious metals and minerals.  And their pelts are highly valued, both for their beauty and for their ability to provide complete immunity to radiation.

Mutations:  Energy Ray (Radiation) (Modified), Reflective Epidermis (Radiation), Toxic Weapon (Venom)