The Cat-Sìth is, perhaps, the most unusual fairy in all of Scottish folklore. The Cat-Sìth, found both in Scottish and Irish mythology, is said to be a fairy with the form of a large cat, often as large as a dog. The Cat-Sìth is black all over, apart from a white spot in its chest. Legend says that the large cat walks on all fours just in front of humans; if human eyes are not watching, the Cat-Sìth walks only on its hind legs. Around humans the Cat-Sìth appears with its back arched and its fur erect, as if to show her dominance.

The Cat-Sìth, however, is not unanimously and uncompromisingly considered a fairy. Due to its ability to walk on just two of its legs, some local variations of the Cat-Sìth legend suggest that the creature is no fairy, but, indeed, a witch! In further development of the witch theory, some people would argue that the Cat-Sìth is a witch that could transform into a cat and then back into her human form just nine times. If the witch were to turn into a cat for the ninth time, she would remain like so for the rest of her days.

No matter her true nature, either as a fairy or as a witch, the Cat-Sìth was generally considered a malevolent creature and ought not to be trusted. It was believed that the Cat-Sìth could steal the souls that had yet to make the passage from the land of the living to the land of the dead, so they would hover around places where burials were about to occur, hoping to snatch the still wandering souls. There was, thus, a special guard put into place called Fèill Fhadalach, that would watch over the soon-to-be-buried bodies and the souls they contained. They played games, and wrestled, and played music, and danced, and told riddles in order to distract the Cat-Sìth and keep them away until the time of the burial.

The day of Samhain is generally considered a special day within the Cat-Sìth lore. A saucer of milk would be left outside each household for the Cat-Sìth, as it was believed that, by leaving such an offering, the Cat-Sìth would take a liking to the household and its members and bless them for the coming year. If, however, such an offering was either forgotten or intentionally omitted, the Cat-Sìth would curse the household: just as she had had no milk left for her, the household should have no milk either, and the udders of all the cows the family owned would dry.

Nowadays, it is commonly accepted that the legend of the Cat-Sìth, especially with regard to their appearance, is based on the Kellas cat, a hybrid between domesticated cats and the wildcats of the Scottish Highlands. In addition, the Cat-Sìth are now commonly recognized as the origin of plenty of superstitions and mythical associations, like the idea of cats having nine lives, or even the association of black cats with magic, Halloween, and being harbingers of bad luck.