Why, though?
What does this Faerie Dragon, typically only associated with druids and the Dream, have to do with the Legendary questline, the final raid tier, or the expansion as a whole? It simply doesn't fit any part of the theme of Mists at all. If there was going to be a mountlike proc or effect from the Legendary like Tarecgosa, wouldn't it make more sense for it to have to do with the Celestials or Wrathion? Or even Seige?