Quote Originally Posted by Urthdigger View Post
It's along the main storyline.
Well, not directly. But joining a Grand Company is part of the main scenario, and once you've done that, then you can take the side quest "My Little Chocobo" to get your chocobo mount (and unlock the mount system in general for any other mounts you get).

You pick up the quest from one of the NPCs at your GC headquarters. Then, as mentioned, you collect Grand Company seals (2000 of them if they haven't nerfed it since I last checked) and in exchange, you'll be issued a chocobo. (There's no licence involved, though.)