"Sidhe" (pronounced, "shee") are a race of fey from Celtic mythology, who live below the ground. I think they're generally described as humanoid, though. The term has come into more common parlance, however, due to the ubiquity of "Cait Sith" (actually pronounced "caught shee"), the mythical "prince of cats" rumored to live in the Scottish highlands. I understand the character makes numerous appearances in the Final Fantasy and other JRPGs, making it by far the more visible concept. The "cat" part is probably what influenced the artist here.