When I re-started playing this game last summer (welcome back campaign) (played for 2 weeks @ launch and gave up), I quickly learned about the Legacy campaign.
Being an utter newb with no faith in ever obtaining a chocobo of my own, I was most excited about this Legacy Chocobo mount! It was actually my "reason" for grabbing a 90 day subscription before they stopped selling them. Crazy, right? (Little did I know that it would be over a year before any of this would finally come to an end.) Of course, I did end up getting my own chocobo (named Dobby) making lots of friends, joining a couple of great linkshells, and getting completely and utterly hooked on this game.
I can't thank that Legacy Chocobo enough for being the odd thing that "lured" me into Legacy, something that ended up having more rewards than many of us could have ever imagined. I'll need to start thinking of a good name for it.
As for the OP question, I agree with Nix. I think it will be delivered to us via moogle mail on release day, with all the other items we will be entitled to (CE items, pre-order items, previous items like garlond goggles, etc.)