Mudman

A pool of muddy water begins to take shape. The mud animates into a roughly humanoid shape with four thick fingers on each hand and it's body trailing off into the dirty pool below. It's eyes are jet black. 

Source: Monstrous Manual AD&D 2nd Edition

     Mudman are unintelligent lifeforms that are created when magic has corrupted a water supply. Residual magic that has leaked into a pool will sometimes spawn a Mudman. The Mudman has only two states of activity. In one state they simply rest and blend into the pool they inhabit. When disturb by a trespasser they spring into activity by forming a semi-humanoid shape to assault the intruders. 

     When in their dormant state they are virtually undetectable. They spread out on the bottom of a pool and siphon the arcane energy that has saturated the surrounding terrain. When in this state they are immune to all non-magical and magical weapons. Spells however will still effect them. When aroused it takes one full round for the Mudman to form a body to defend it's lair.

     Mudmen are not natural creatures. Their only purpose is to kill those who invade their pool. They do not hoard treasures. Any leftover treasures from a previous kill are slowly absorbed into it's form. Recent treasure that is less then one month old will have not been absorbed. 

Combat: Mudmen are small creatures who appears in groups of 2 - 12. They have an experience value of 175. 

-Mud-Throwing: Mudman hurl mud at their opponents. They treat their victims AC as 10 and each hit reduces one's movement by one. Each round they move towards their victim and when they are within 10ft they hurl their bodies on their target. On a successful hit the Mudman has sacrificed it's body and is dead. Once the victim is engulf it's movement is reduced by 4. It takes five rounds to remove 1 point of movement penalty due to the mud.

-Suffocation: Between hurling mud and engulfing victims the threat of suffocation is real. If an opponent is reduced to zero movement they begin to suffocate. Each round they take 1d8 points of damage until their nose or mouth is cleared. If five rounds go by without having their airways cleared the victim dies. 

-Spells: Mudman takes damage normally from spells that harm a living creature. The spells dig and dispel magic act as fireball against the Mudman. Transmute mud to rock instantly kills them.

-Immunities: Mudman are immune to poisons and are unaffected by spells that target the mind.

     The most interesting mechanic of the Mudman is the movement reducing mud. Having multiple Mudmen attack the party can wreck havoc. Especially if a halfling or gnome is targeted. Since they have the lowest movement rate they begin suffocating within a couple of rounds. I like the fact that Mudman have a kamikaze ability. I don't think their are many creatures that intentionally kill themselves.