The Barrow-Wights are part of J.R.R. Tolkien's Middle Earth mythology.
One of the books in the pile that Slimer accidentally topples is called "Barrow": the Barrow Wights later appear in the episode.
The voice actors were recorded on December 17, 1986.
Egon and Ray compare Barrow Wights to Trolls and Vampires, both of whom of the Ghostbusters have faced before.