Cait Sith has been named after that mythological creature.
Orthography accepted are: Cat Sìth, Cat Sí, Cat Sidhe, Cath Sith, Cait Sidhe, and even Fairy Cat.
Pronounced [kʰaʰt̪ ˈʃiː] or "Cat Shee".
So I think the Japanese got it pretty right by saying "Ket shi". :)
"Cait Sith" isn't in the list...