Deadites are one of the monsters seen on the whiteboard. They were bet on by the Story Department.
In the Evil Dead, Deadites are people who have become possessed by a demonic or evil spirit, transforming them into an undead monstrosity. The anatomy of a possessed person alters slightly after becoming possessed; the teeth grow sharper, eyes turn pale, and skin decays slightly. The longer someone remains possessed by a demonic spirit, the more hideous they are in appearance. It is never revealed whether the Deadites in The Cabin In The Woods function in this way as well.
A tape recording with verses from the Necronomicon summons them just as it did in the original Evil Dead and hinted towards during the end credits of Evil Dead 2013.
Its possible the deadites were a hint towards the 2013 remake/reboot/continuation of Evil Dead.
The Deadites are a direct reference to the Deadites of the Evil Dead series.