Always this kind of speculation when there's a new mount on the PTR, how great would it be if this was the reward for having 150 mounts? Seems like one model is the NPC and the other is the mount, they could make it so you are teleported to somewhere and need to defeat the Chromatic Dragon in order to obtain him as a mount.
With all the new mounts datamined for this patch, it does seem like Blizzard are considering a 150 mount achievement.