To answer your question, the Reins of the Grand Expedition Yak is account-wide.

With the exception of PvP reward mounts, all mounts are account-wide. Mounts that a character is eligible to use will be displayed in your Mount Journal. Certain mounts have profession/faction/class requirements and will only be available for characters that meet them.

As an example, Horde/Alliance faction mounts will only show up in the Mount Journals for characters of that faction.

If you’re interested in learning more about what types of mounts, pets, or achievements are account-wide, check out this knowledge base article.

10/02/2012 04:25 PMPosted by Vail

With the exception of PvP reward mounts, all mounts are account-wide.

Untrue.
Cloud Serpents. You need to grind the reputation to buy the cloud serpent license or some other nonsense. Hope that gets changed.

'Least thats how it was the last time I looked.

Cloud Serpent mounts are indeed account-wide. However they require the Cloud Serpent Riding skill, which is a faction requirement and character specific. Characters that meet the faction requirement will see the mount as account-wide. I’ll bring this up with our developers to see if that’s intended.