Unicorn

The Unicorn is one of the monsters seen on the whiteboard. It is shown that the Engineering Department bet on it.

Physical Description & Attributes

It's shown as a white horse with a large horn on its head.

It is seen during the Purge when it chases and stabs 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, maybe 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 Rumplestiltskin, Alice\Alice Madness Returns,Hansel And Gretal: Witch Hunters and Gingerdead man. Some people have argued that maybe this is a metafiction 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.