Familiar spirits

As I am already in a Halloween mood, I have started reading horror stories. Among my reads is In a Glass Darkly, the classic collection of short stories from Sheridan le Fanu, an author I had long wanted to revisit. One of the stories, The Familiar, I had wanted to read since my teenage, when I had borrowed a book from the library that included the story, but had to return it before I could start it. I was suspecting that the familiar from the title was a familiar spirit, the supernatural entity that helps witches do their evil deeds. It turned out that it was the ghost of the story, which at some point may or may not have taken the guise of an owl that was the pet of the fiancée of the protagonist. Maybe the owl was a familiar after all...

In any case, familiar spirits would make for great antagonists in horror stories. They have not been used much, from what I know (i.e. I have not seen them in anything I can think of, except this vague implication in Le Fanu's short story). Whether they are ghosts or demons, they represent a far more tangible menace as they have a physical form, they are not merely apparitions. There should be stories written about them, especially if they take the already ghostly and spooky form of an owl. Any ideas? You can write them in the comments below. Oh, and I took this picture in an owls sanctuary in Derbyshire. I have no idea what kinds of owls they are, but they looks very sinister and could make very convincing familiar spirits.