What you originally thought was a patch of muddy water reveals itself to be a protoplasmic life-form resembling a giant brownish-black amoeba.
Mudbog CR 3
Speed 10 ft.
Space 10 ft.; Reach 0 ft.
Special Attacks acid, engulf (1d6 bludgeoning plus 1d6 acid damage, AC 4, hp 6)
A mudbog secretes a digestive acid that quickly dissolves organic material, but not metal or stone. Any melee hit deals 1d6 points of acid damage. Armor or clothing dissolves and becomes useless immediately unless it succeeds on a DC 17 Reflex save. A wooden weapon that strikes a mudbog dissolves immediately unless it succeeds on a DC 17 Reflex save. The mudbog’s acidic touch deals 17 points of acid damage per round to wooden objects, but the ooze must remain in contact with the object for 1 full round to deal this damage. The save DCs are Constitution-based.
The consistency and coloration of a mudbog makes it difficult to discern from a normal patch of mud. Any creature within 10 feet of a mudbog must make a DC 20 Perception or Survival check to determine the actual nature of the creature.
A Medium or smaller opponent that walks into a mudbog is automatically grappled and takes 1d6 acid damage each round the mudbog maintains the hold. Engulfed creatures are considered to be grappled and are trapped within the mudbog’s body. This ability otherwise functions as swallow whole, save that a creature that cut its way out of a mudbog leaves no hole in the protoplasmic creature’s body.
Treasure double standard
Mudbogs are strange, slow-moving, pudding-like creatures that dwell in swamps, fens, and wetlands. Situated comfortably in its hole along well-traveled paths, a mudbog is likely to be mistaken for a patch of muddy water or a wet spot in the road. Creatures attempting to simply walk over or through the “mud puddle” find themselves stuck fast and, worse yet, being consumed by the hungry ooze.
A mudbog doesn’t really attack its opponents; its method of getting a meal is strictly passive. However, once a meal stumbles into its protoplasmic body, the mudbog is determined not to let it escape. They are brownish in color, resembling nothing more than brackish mud. These monsters use their coloration to their best advantage and use their acidic bodies to dig a hole, usually several feet deep, in which to lie in wait for a meal. The average mudbog is roughly 10 feet across and 3 feet deep.
A mudbog lacks the intelligence to collect treasure. However, the possessions of its victims can always be found in a heap at the bottom of the hole it has chosen as its home. Actually getting to the treasure involves somehow removing the mudbog from its hole.
Mudbog from the Tome of Horrors Complete, Copyright 2011, Necromancer Games, Inc., published and distributed by Frog God Games; Author Erica Balsley.