Sluagh are a breed of supernatural creature, introduced in Season 5. They have a rather grotesque appearance, with a tea green hide, a pair of pointed ears and a mouthful of fangs.

 

Sluagh have the ability to glamour themselves in others' vision depending on who lays eyes upon the creature. Mythology says sluagh take on the appearance of "lost souls" that have become bound to the creature.

 

One particular sluagh is a prisoner at Eichen House's secret closed unit for supernatural prisoners.

 

In A Novel Approach, Stiles lays eyes on the demon in its cell, but at first sees him as Donovan Donati, whom Stiles had just killed by accident and in self-defense, and was traumatized by the deed. However, as he walked further on and got a closer look, he saw the sluagh in its true form.