The Swamp Creature is a fish-like monster created by Dr. Wily.
Dr. Wily sent the Swamp Creature to Bayouland to eat Kevin Keene when he went looking for Duke, but it found Princess Lana and Simon Belmont first. It chased them through the swamp until they were caught by King Hippo and Eggplant Wizard, who also captured Kevin when he tried to save his fellow N Team members. Luckily, Bayou Billy showed up with Duke and distracted Mother Brain's minions while Kevin dealt with the Swamp Creature. It shorted out his Zapper with swamp water, but he was able to destroy it with a crash star.