Mutant swamp beasts are enemies in Going Commando encountered in the Megacorp Outlet area on planet Oozla. They are medium sized beasts that have the body of a large snail, a single eye stalk, and four tentacles with crab-like claws that swipe at enemies by plunging to attack them. It was theorized that they were hatched by a larger 'Mother Beast', which was later discovered to be the swamp monster II, an immense version of the swamp beast. Ratchet encountered them during "Find the store entrance" (in which they killed many lone Megacorp employees), "Investigate the Megacorp store" (in which they had infested the outlet itself), and "Explore the swamp ruins". They were often fought alongside the mutant muckdweller and mutant fireflies.
Mutant swamp beasts are often found swimming in the water before jumping to attack Ratchet once he approached, or otherwise circling an area. They attack by swinging all their tentacles in front of them to attack with their claws, and if they land a hit, they clap their claws together and applaud themselves. Mutant swamp beasts are relatively tough enemies, but are slow enough to be avoided as they can only attack in melee range. The Lancer and Chopper are both effective against them, as they should be fought from a range. In clusters, the Gravity Bomb can defeat many at once, though the ammunition is wasted on a few. Also, if Ratchet shoots a swamp beast's eye stalk while it is in the water, it will instantly die in a single hit.