The Highland fairy cat. J. G. Campbell describes it as being as large as a dog, black with a white spot on its breast, with an arched back and erect bristles. This, probably, would be when it was angry. He says that many Highlanders believed that these cats were transformed witches, not fairies. 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. But Big Ears was a monstrous demon cat who had only a slight connection with the Cait Sith.
In reality, the Cait Sith is almost certainly what is now known as the Kellas Cat - a large black hybrid between feral domestic cats and Scottish Wildcats. Specimens of these impressive hybrids were examined scientifically in the 1980s.
Cait Sith is also a game character from the RPG (Role-Playing Game) called "Final Fantasy 7". It's an excellent game. Anyway, Cait Sith (pronounced: Cat Shee) rides around on the back of a huge stuffed Mog he magically brought to life. He holds a Megaphone in his hand, which is his weapon(!) and he's always shouting orders and creating dopey attacks. His hobby is fortune telling, but like his personality, it's pretty unreliable.