The cat sith comes from Celtic legends. It was a fairy creature that looked like a black cat with a white spot on its breast. It was said to haunt the Scottish Highlands. It was thought that if it passed over a corspe before burial it could steal the persons soul before the soul had a chance to be claimed by the gods. Corpses were watched night and day to protect them. Distractions such as games, catnip, riddles, and music were used to keep the cat sith away. Fires were not allowed in the room with the body because it was thought the cat sigh was attracted to warmth.

On Samhain it was thought that if you left out a saucer of milk for the cat sith that it would bless your house. But not to do so would lead to a curse of having all your cows' milk run dry.

In some stories the cat sith was a witch who had the ability to change back and forth between a human and a cat eight times. If they changed a ninth time they would remain a cat forever.