The Fat Chocobo is a rare and noble breed of chocobo. Before retainer's existed they were the ones who would allow the heroes to store items to clear space in their inventory. Without them the heroes would of been to encumbered to defeat the final boss when you consider some were to lazy to barter/sell their old gear when gaining new armor and a majority of heroes were hoarders in the fact that they would just pilfer any items they could from chest, defeated creatures and stock up heavily on potions as needed.