Blizzard has confirmed that chapter 4 and 5 of the Patch 10.1 campaign unlocks next week and includes the Slitherdrake mount.
Venethash: Can we get a confirmation please Yes! Chapters 4 and 5 of the questline will arrive next week, and that will include the Slitherdrake mount.We also have some updates to in-game text that should clarify this coming soon.. Read more on wowhead.com