Familiar (Rat-Thing, Tilberi, Homunculus, Imp)
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"
A Rat-Thing is a ratlike creature with a human face and hands. First presented in "The Dreams in the Witch House" 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
A Tilberi is a familiar creature from Mythic Iceland, created by witch-women from the rib of a freshly-dead human corpse wrapped in wool by the light of a full moon, warmed between her breasts for three weeks, and then suckled on blood from her 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. The Tilberi can speak in languages known to the corpse and/or witch, and can be instructed to go forth into the night to run simple errands for the witch, usually involving simple acts of theft (milk, wool, small objects), delivering messages, spying on others, or committing minor acts of mischief and mayhem.