The Cat Sìth (Scottish Gaelic: [kʰaht̪ ˈʃiː]) or Cat Sidhe (Irish: [kat̪ˠ ˈʃiː], Cat Sí in new orthography) is a fairy creature from Celtic mythology, said to resemble a large black cat with a white spot on its chest. Legend has it that the spectral cat haunts the Scottish Highlands. The legends surrounding this creature are more common in Scottish folklore, but a few occur in Irish. Some common folklore suggested that the Cat Sìth was not a fairy, but a witch that could transform into a cat nine times.
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 Scottish wildcats and domestic cats only found in Scotland (the Scottish wildcat is a subspecies of the European wildcat, which is absent from elsewhere in the British Isles).
Typical Kellas cats (pictured here) 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 two millennia or more. Others believe that the Cat Sìth was inspired by the Scottish wildcat itself.
In the British folk tale The King of the Cats, a man comes home to tell his wife and cat, Old Tom, that he saw nine black cats with white spots on their chests carrying a coffin with a crown on it, and one of the cats tells the man to “Tell Tom Tildrum that Tim Toldrum is dead.” The cat then exclaims, “What?! Old Tim dead! Then I’m the King o’ the Cats!” Old Tom then climbs up the chimney and is never seen again.