What you originally thought was a patch of muddy water reveals itself to be a protoplasmic life-form resembling a giant brownish-black amoeba.
Speed 10 ft.
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.