The Cat Sìth or Cat Sídhe is a monstrous fairy cat from Scottish and Irish mythology.
The root words Cait (meaning cat in both Irish and Scottish Gaelic) and Sidhe, that stands for faery folk and/or other otherworldly beings, means fairy cat.
J. G. Campbell describes it as a huge black cat being as large as a dog, with a white spot on its breast, with an arched back and erect bristles.
Just like a real cat, Cait Sith could be ferocious if stumbled upon.
Many Highlanders believed that the Cat Sìth were transformed witches, not fairies.
As proposed by British cryptozoologist Karl Shuker, in his book Mystery Cats of the World (1989), it is possible that the legends of the Cat Sìth were inspired by Kellas Cats, which are probably a distinctive hybrid between European Wildcats and domestic cats only found in Scotland (the European Wildcat is absent from elsewhere in the British Isles). Typical Kellas Cats resemble large black wildcats, but with some peculiar features closer to domestic cats, and have probably been present in Scotland for centuries, maybe even some 2 millennia or more.
Other black cats
A not dissimilar creature appears in Edgar Allan Poe's short story, "The Black Cat", in which an ominous feline appears with a white patch on its breast whose shape appears to change into that of the gallows as a means of exacting vengeance on its master for its predecessor's death.
An even larger and more ferocious cat, the demonic god of the cats, appeared in answer to the wicked and ferocious ceremony of the Taghairm, which consisted in roasting successive cats alive on spits for four days and nights until Big Ears appeared and granted the wishes of the torturers. The last ceremony of Taghairm was said to have been performed in Mull and was described in detail in the London Literary Gazette (March 1824). The account is quoted by D. A. Mackenzie in SCOTTISH FOLK LORE AND FOLK LIFE.