Casey Kasem, who used to play the original Shaggy, plays this incarnation of Shaggy's father in this episode. Mr. Kasem is a strict vegetarian and had suggested Shaggy should be a vegetarian as well. In this episode however, when this version of Shaggy is forced to eat vegan he doesn't like it at all.
The name Howard E. Roberts is a play on the American pulp fiction writer Robert E. Howard, most famously known for creating Conan the Barbarian.
This episode is full of references to H.P. Lovecraft a horror writer (famous for this type of monster) and his works, namely "The Call of Cthulhu":
The name "H.P. Hatecraft" is a play on the name H.P. Lovecraft.
Char Gar Gothakon himself bears a strong resemblance to Lovecraft's most well-known creation: Cthulhu.
The episode title may be a reference to In the Mouth of Madness, a film in which Lovecraftian horrors come to life out of books.