Familiar (Rat-Thing, Tilberi or Snakkur, Homunculus, Imp, Tulpa); common names include Brown Jenkin (rat), Greymalkin (cat), Paddock (frog)
A familiar spirit or creature is created as a servant by a witch, usually in some twisted, shriveled, and monstrous parody of the life of a victim it was created from, perhaps a cultist or victim sacrificed by the witch. A prototypical example from weird fiction would be Brown Jenkins, the "Rat-Thing" familiar from a Lovecraft story, but such spirits can take other forms.
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 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 (fiction)"
A Rat-Thing is a ratlike creature with a human face and hands. First presented in "The Dreams in the Witch House (fiction)" as the familiar Brown Jenkin, the idea was later generalised to a whole type or race of beings.
Rat-Things are created through magic, transforming humans (typically cultists) into servants. They often have some knowledge of magic.
In Call of Cthulhu
Tilberi and Troll-Cats
A Tilberi ("carrier" or Snakkur "spindle") is a familiar creature from Mythic Iceland, created by witch-women from a large straight bone from a freshly-dead human corpse wrapped in wool by the light of a full moon, warmed between her breasts for three weeks during which the witch feeds it mead or wine that has been blessed by a Pagan or Christian priest by spitting the alcohol on the Tilberi. By the second week, the Tilberi begins to move (which can attract attention if the witch is not careful), and by the third week it comes to life, after which it must be suckled on blood from it's creator's witch's teat (a mole or cut, usually on the witch's thigh). The Tilberi resembles a woolly creature with two heads, one at each end of the bundle, and sharp teeth in tiny and monstrous faces resembling that of the corpse. When at home, the Tilberi clings to the witch's teat and drinks the witch's blood. The Tilberi can speak in languages known to the corpse and/or witch in a thin, childish voice, and can be instructed to go forth into the night to run simple errands for the witch, usually involving acts of petty theft (milk, wool, small objects such as coins or jewelry), delivering messages, spying on others, or committing other minor acts of mischief and mayhem. If the Tilberi's mistress dies before it returns from its mission, the Tilberi might go insane, feeding on the blood, hair, and wool of any farm animals or people it encounters, rolling in sheered or ripped-out hair and wool to form a ball and growing to prodigious size, until it is big enough to begin consuming corpses and killing victims for food. The Tilberi, in drinking her blood, drains the witch a little more every day until she eventually dies, unless the Tilberi is killed; the only ways to kill a Tilberi is for its mistress to order it to undertake a very difficult task that it will work itself to death to complete (such as eating all the droppings in a large sheep pasture), or to burn the witch and Tilberi together at the stake.
A similar creature is the "Troll Cat" or "Dust Bunny", a familiar which usually appears in the form of a large ball of wool, hair, wood shavings, and/or finger/toe-nail parings, or sometimes in the form of a woolen cat or rabbit, shaped by the witch and animated with witch's blood, which otherwise behaves similarly to a Tilberi.