I think putting the letters in those cells diagonally is better, thanks.
Demeter, I think describing the creature as mythical or legendary (as in the preamble) is overstating it. I would call it fictional, in the same category as the owl and the pussycat who went to sea (though it has nothing to do with Lear). And it is obscure, so hardly legendary. In fact I'd only heard of it before because it was the subject of another crossword like this a few years ago, possibly an Inquisitor or Enigmatic Variations.