Merman

The Merman is one of the Monsters kept in the Facility. He is listed on the Whiteboard and is bet on by Steve Hadley, as he has never seen the Merman before. He was released during the System Purge but is only seen at the end of the sequence.

Physical Description and Attributes
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. The origins of the Merman are unknown but its likely he has been around for quite sometime, given merfolk only exist in mythology.

If the Merman were to be summoned from the Cabin, its likely his Cube Prison would be brought up underwater in the lake nearby, as he is more effective in water. It can be assumed that the Organization would release the Merman into the water while the victims were swimming or flood the land surrounding the cabin to allow him greater access to his victims, as the Facility has a great deal of control over the environs of the cabin.

Curt Vaughan almost summoned the Merman by blowing the conch shell in the cellar.

Appearances
The Merman is not seen in his Cube Prison. Later, he does appear during the System Purge, where he finishes off an injured Steve Hadley during the breach of the Control Room. This is the only time we see the Merman during the Purge. This scene is ironic in that the first and only time Hadley sees the Merman is when he is killed by him.

The Merman has a large segment in the behind the scenes footage, where the crew and special effects team highlights the system in which the blood was blown through the Merman suit's blowhole.

Trivia

 * 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 actor who plays the Merman, Richard Cetron, also plays The Werewolf.

Inspiration
The Merman is likely inspired by The Creature from the Black Lagoon or the cult classic Frankenfish. Piscine amphibious humanoids (known as mermen 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.

The Merman in the film is very similar to the Merrows from Irish folklore. 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).

Humanoid creatures very similar to mermen and mermaids populate modern written horror, such as the aquatic Deep Ones of H.P. Lovecraft's The Shadow out of Innsmouth, the story Creature by Peter Benchley and Mira Grant's Rolling in the Deep series.