In the first book of the fantasy science fiction Pendragon series, Pendragon: Merchant of Death, there are beings called Quigs. In the book Quigs serve as the guardians of the flumes. These are portals that travelers use to move between territories. The flumes are later destroyed in an attempt to keep them from being used for evil.
The Quigs attack anyone who attempts to enter the flumes including travelers. These creatures are under control of Saint Dane, the main antagonist of the story. They only take form when he is near. The Quigs can be stopped with a dog whistle.