Public Service Announcement: Using Thavnairian Onions Correctly
We've had a few unfortunate misuses of Thavnairian Onions in our FC due to the whole system being ridiculously unclear, and the criminal lack of safety features to prevent incorrect use.
I put together this little diagram showing the Chobobo exp bar from the Companion menu (or View Details inside stable):
http://gondwana.inzoum.com/images/thavnairian_onion.png
It's a common misconception that Thavnairian Onions are required to make your Chocobo rank up. That's not how they work. They let your Chocobo resume earning exp towards the next rank.
Update:
Alice_89th points out that you must Train your Chocobo with a Thavnairian Onion yourself, as Chocobos trained by another member will not see their rank cap increased.