In Scottish folklore, the large black fairy cat with a white spot that is treated as an omen is the Cat Sìth.
The Cat Sìth is usually described as an unusually large black cat with a white patch on its chest. Some stories say it walks on its hind legs when people are not watching. Its name means “fairy cat” in Scottish Gaelic, while the related Irish form is cat sí.
The creature was sometimes believed to steal the souls of the dead before burial, which helped inspire the practice of keeping watch over a corpse. At Samhain, people were also said to leave milk outside for the cat Sìth, hoping to gain good fortune or prevent mischief.
A common mix-up is Cù Sìth, which is a supernatural dog rather than a cat. Folklore also sometimes portrays the Cat Sìth as a witch able to transform into feline form, not simply as a fairy creature.