Final Fantasy XIV: Heavensward added in something very special when it came out. Aside from the great addition to the story, they gave us the ability to fly about the different locations of Hydaelyn with customizable winged mounts. The process to create those and really integrate them fully into the greater game was a bit more difficult than you'd think, and that process of animating the mounts is detailed in the newest Developer Diary video from Square Enix.
It was a challenge to make sure that flying was a smooth process that didn't detract from the beauty of the game. The designers had to decrease the overall amount of data while flying so there wasn't any stuttering. That and adding in an element of elevation was tricky, you can't have the controls be jerky or unintuitive.
The most important part, naturally, was introducing the Fat Chocobo as a potential aerial ride. But how does one make a gigantic creature like that capable of flight? Well, with the right motivation; cake. Check out the video, it's a very enlightening look into the creative processes that they go through to bring things to life.