Three Headed Dog of Doogenkirk

 The Three Headed Dog of Doogenkirk is a monstrous three headed dog which appears in Prunella's fantasy in the introduction to Prunella Packs It In. It guards a castle containing a magic chalice and Prunella (as "Grizelda the Fearless") tries to sneak past it. The dog may be a character from the Henry Skreever book "The Chalice of Malice". It may be a reference to Fluffy from "Harry Potter and the Philosopher's (Sorcerer's) Stone".