Swamp monster I is the first boss in Going Commando. It is an immense creature encountered in the Megacorp Outlet area on Oozla. The creature is mostly submerged under the swamp water, and is encountered in a lake close to the outlet. Only its large eye stalk and two of its tentacles are visible, though it likely has an appearance similar to the swamp monster II, which would make it a giant mutant swamp beast.
Swamp monster I was encountered by Ratchet during "Find the store entrance". The monster preyed on Megacorp employees remaining in the area, and attempted to stop Ratchet before reaching the area. Ratchet defeated it before moving onto the outlet. He later defeated swamp monster II, an apparently related monster living in the caverns underneath the outlet, after obtaining the Gravity Boots.
Behind the scenes
After killing swamp monster I, the Monsterpedia will list three of them as dead instead of one. This is likely because the game counts the three components separately.