|Not quite as big as a dog... My cat Shadow|
I first came across the character Cait Sith in New Vampire Miyu by Narumi Kakinouchi. I don't know if she intended for him to be based on the Celtic legend or not.
Anyway, a few years ago I was working on a YA Sith novel & came across the name "Cait Sith" again. (Note: Sidhe is a variant spelling.)
The legend of Cait Sith is apparently Scottish in nature and refers to a large cat from the Highlands. This creature, according to the Scottish folklore site, is "as big as a dog and completely black except for one white spot on its breast." (http://www.mysteriousbritain.co.uk./folklore/scottishfolkapp.html)
In my untitled Sidhe novel, Cait Sith/Kellas Cat is a protector of the heroine and also a Threshold Guardian (for you Writers Journey fans). Hm, maybe I should revive this story.