Advantages

Extrapolating on the chicken resemblance, we can surmise that a chocobo-like mount would have several advantages over a horse:

• Size - standing on just two feet, a chocobo takes up half the room.
  
• Temperament - horses are notoriously angry creatures, whilst chickens are, by and large, nothing other than laid-back (and occasionally flustered).
• Softness - anyone who has picked up a chicken will be aware that it is like picking up a cloud. Riding a chocobo to work, you could be mistaken for thinking you were still in bed. Comparatively, horses are like rib cages wrapped in sheathes of hardened muscle and gristle. Nobody has ever made a pillow or a tickling stick out of a horse.
  
• Free food - a chicken will periodically produce free food. A chocobo will periodically produce giant free food.