Unicorn

The Unicorn is one of the monsters kept in The Facility. It is also seen on the whiteboard with the Engineering Department betting on their release.

Physical Description & Attributes

It's shown as a typical unicorn, a white horse with a large horn on its head. It is seen in it's containment cube, lying down and waiting for it's release, and during the Purge when it chases and impales a worker with its horn.

There is a Unicorn Tapestry in the basement. If it is tampered with, it will summon the unicorn.

Inspiration

Unicorns have appeared in horror films before, however, it's possible Drew Goddard and Joss Whedon put it in as an inside joke and/or an extremely sarcastic note on how many other mythical creatures and fairy tales have been used in horror filmmaking as evil ones examples being Rumpelstiltskin, Leprechaun; the video games American McGee's Alice and it's sequel Alice: Madness Returns; Hansel & Gretel: Witch Hunters and Gingerdead Man. It is also thought to be a reference to Supernatural's use of a unicorn in the episode Plucky Pennywhistle's Magical Menagerie (#7.14), however the release date is only a few months later than this episode. Some people have argued that it may be a metafictional note on the fakeness of the scenario, linking it to the famous origami unicorn in Blade Runner.

Unicorns, despite their majestic and approachable nature in today's fiction, are actually known in lore as being deceptively violent in nature. Traditionally, a unicorn would be drawn to the purity of a virgin, but if one was merely pretending to be a virgin, a unicorn would actually kill that person.

It may also have been inspired by this:

Aptly named, too.

Additionally, unicorns have appeared a few occasion into dark fantasy movies like Black Moon (1975) and Legend (1985).