The Merman is one of the monsters on the whiteboard. He is bet on by Hadley. He laments on the Merman not being chosen, as it is one of the few monsters he has never seen.
The Merman is shown as a half-human, half-fish monster with flabby blue skin, a large mouth full of sharp teeth, long unkempt black hair, and a blowhole on its back. It kills by attacking the victim with its teeth and blowing their blood out of its blowhole. Sitterson comments on this gruesome process, stating that "the cleanup on them is a nightmare." While it can survive on land, it is incredibly slow and bulky and thus only effective at finishing off incapacitated victims.
It is summoned by blowing on the Conch Shell in the basement.
The Merman is one of the only monsters not shown in its cube cell. Later he does appear during the System Purge, where he finishes off an injured Steve Hadley during the attack on the control room. The Merman was among the monsters portrayed in the Universal Orlando Attraction Halloween Horror Nights 23 in 2013. In this appearance, though, it was not portrayed by a person wearing a costume as it was in the film, but instead as a life size, non-moving prop.
The Merman is likely inspired by The Creature from the Black Lagoon or the cult classic Frankenfish. Mer-men and mermaids (or mer-folk in general) are prevalent in nearly all the world's mythologies, particularly in Greece whose sea-based culture described sea divinities, such as Oceanides and Nereids. The Sumerians worshipped Atargatis, the mermaid-goddess, but the Japanese feared Isohime, a giant mermaid who snatched and drowned sailors on the sea.
Russia is said to have derived its name from Ros, the mermaid-goddess. In classical mythology, mermaids were beautiful fish-tailed women who abducted sailors as lovers while mer-men were usually described as ugly. However, most scholars believe these legends were based on animals like manatees and dugongs unfamiliar to fishermen. Mermaids appear often in modern media, such as the "Chicken of the Sea" mascot and the Disney motion picture, The Little Mermaid (1989).