A Rat-Thing as portrayed in H. P. Lovecraft's story "The Dreams in the Witch House" is a rodent-like creature with a face and hands resembling a human's. It is a Mythos entity similar to the traditional witch's familiar. The concept was later generalised to a whole type or race of beings.
A familiar is a spirit or creature created as a servant by a witch, usually in some twisted, shriveled, and monstrous parody of the life of the person it was created from, perhaps a cultist or sacrificial victim. These entities can traditionally take other forms, such as bats, birds, cats, dogs, ferrets, frogs, hares, and toads. Examples of familiars names include Greymalkin and Vinegar Tom (cats), Jarmara (dog), Paddock (frog), and Sacke & Sugar (hare). The archetypal example from weird fiction is Brown Jenkin, the "Rat-Thing" familiar of the witch Keziah Mason.
Rat-Things and other familiars often have some knowledge of magic, either as an extension of their natural abilities or through instruction from the witches and cultists that create them.
"Witnesses said it had long hair and the shape of a rat, but that its sharp-toothed, bearded face was evilly human while its paws were like tiny human hands. It took messages betwixt old Keziah and the devil, and was nursed on the witch’s blood—which it sucked like a vampire. Its voice was a kind of loathsome titter, and it could speak all languages."
"The horror would appear to pop out of the rat-hole in the corner and patter toward him over the sagging, wide-planked floor with evil expectancy in its tiny, bearded human face—but mercifully, this dream always melted away before the object got close enough to nuzzle him. It had hellishly long, sharp, canine teeth. […] Its shrill loathsome tittering stuck more and more in Gilman’s head, and he could remember in the morning how it had pronounced the words “Azathoth” and “Nyarlathotep”."
"The bones of the tiny paws, it is rumoured, imply prehensile characteristics more typical of a diminutive monkey than of a rat; while the small skull with its savage yellow fangs is of the utmost anomalousness, appearing from certain angles like a miniature, monstrously degraded parody of a human skull."
- H.P. Lovecraft, "The Dreams in the Witch House"
Heresies and Controversies
Alternatively, Rat-Things may be portrayed as the transformed bodies of humans (typically cultists) hosting Elemental spirits, which act as servants for witches and wizards. They may also be presented as a physical and stable form of Tulpas, which are more typically formed from the vitality, essence, and attention of their creator, upon which normal Tulpa become dependent for existence. See also the Scandinavian häxan's Tilberi or Snakkur.
- August Derleth and H. P. Lovecraft - The Lurker at the Threshold
- Fritz Leiber - "Through Hyperspace With Brown Jenkin"
- H. P. Lovecraft - "The Dreams in the Witch House"
- Graham Masterton - Prey
- Curse of the Crimson Altar (1968)
- H. P. Lovecraft's Dreams in the Witch-House (2005) (part of the Masters of Horror series by Stuart Gordon
- The Evil Clergyman (part of the 1987 anthology film Pulse Pounders)