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 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. It may also have roots in old sailor's tales of mermen, which by their accounts were always ugly, while the mermaids were considered beautiful.