mount

[Urthdigger]

It's along the main storyline. Once you join a Grand Company during it, you can then pay some seals (guildhests, FATEs, and your GC hunting log are good ways to earn them) to get a mount license.

[Niwashi]

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.)