I think I know the one. It's been in the SB benchmark and it's this huge, white/purple parrot like bird with crazy shaped wings (it's the one there on the bottom of the screenshot, in front of the Sophia hawk ). Has little "ears" on the head, bit like the griffin. I have seen people riding it already, but like the OP, I have no clue where you'd aquire that mount. Has to be somewhere in game, because it's not up on Mog Station.