Gillman of the Blue Lagoon is one of the many enemies encountered in the video-game "Zombies Ate My Neighbors" and is a homage to the Creature From The Black Lagoon - this enemy is largely aquatic but will leap onto the land when the hero attracts its attention and will pursue them across the level map much like any other monster.
Gillman is not unique either, a player can find themselves ambushed by multiple Gillman monsters but they shouldn't pose too much of a threat providing the hero keeps their wits about them and avoids being overwhelmed, the reason of the Gillman invasion is unclear but in many levels you can see toxic barrels near pools of water which suggests that Gillman (and perhaps other monsters) have been mutated from this substance - though this is largely speculation as no official explaination has ever been given, though all evidence seems to point to Dr. Tongue being the main cause of the monster invasion.