I asked this very question in another thread, and no one responded, so I believe the lore behind it is a blank at this stage. It's appearance and the mount's description definitely hint that it existed back before the Calamity, but it's an entirely new mount, and one that even players like me who never levelled that much in 1.0 or didn't get the Legacy opening started ARR with (and hence, never had a chocobo of their own in 1.0). I really hope Ferne or someone else on the writing team can answer this as I'm very curious about it.