Well, to be fair the mount guide description for it is mostly just flavour text with little actual basis in reality, as there are actually too many variables to a player's situation for it to really be the actual player's original chocobo (Legacy players who satisfied the requirements after all regain their actual chocobo anyway in ARR as well as the Legacy Chocobo, which is an entirely separate mount, and then there are 1.0 players like me who never actually had a chocobo in 1.0 in the first place - I never did the Seventh Umbral Era story in 1.0 so I never got a company chocobo until ARR... so... where did this one come from?).

Really, it's just handwaved away about the origins and backstory of the Legacy Chocobo, as it's whole purpose after all was incentive for 1.0 players to stick with 1.0 when SE needed it the most, which we did.

*And as for the 'heartbreak and fantasy', considering it's feathers are blackened and singed, it clearly had a close encounter with Bahamut and lived to tell the tale, so I think we can pretty much guess the rest*