Sluagh is a type of supernatural creature within the Teen Wolf Universe.
The Sluagh, according to Valack, can take on the appearance of the lost souls inextricably bound to it.
The Sluagh, while only appearing on the show once, is notable due to how it came to be a part of the Teen Wolf Universe.
AT&T sponsored a contest encouraging viewers to come up with their own Teen Wolf Creature. The winning entry, from Brooklyn-based artist Jessica Short, was designed and executed as a mask by Teen Wolf crew and written into the Season 5 episode A Novel Approach by Writer Angela Harvey.
Under the Mask
16-year-old Texan Michael Lynch played the fan-designed creature in episode 505. Mike got the part through the Make-A-Wish foundation. His wish was to be an extra on Teen Wolf. Instead, because the timing was just right, he becomes a lasting part of the Teen Wolf Universe as the very first Sluagh to appear on screen.