Here's a nice video from the developers of Guild Wars 2's expansion Path of Fire talking about everyone's favorite mode of transportation: mounts (fast travel is illogical and is not actually transportation, call it my postmodern tendencies).
The devs talk about their goal for mounts, focusing on movement speeds, verticality, water and grass types, art and sound design, the whole gamut.
Their focus on what the player feels is particularly interesting, because it shows that this team thinks outside of the box when it comes to the process of designing these creatures.
Essentially, the mounts aren't just cosmetic pieces, but integral elements to the game's world as well as the player.
Although the video below doesn't a hugely detailed process, it's nonetheless a great opportunity to see what goes into making these critical aides in player utility.
You can read more on their blog post, enjoy.