I mean Whales who will buy the mounts for premium prices not just try to get them as drops from seasonal bosses.Based on the flying pose, its a reskin of the Vulpine Familiar mount that's been on the store for a year or two. So like, 90% of the work has been done because the rig and movements already exist. So I don't think this thing took more than two weeks to put together. If that. And that assumes they don't already have half a dozen of these things already premade and ready for release at pre-determined intervals.
Also WoW only has one kind of subscriber. There's no 'whale' tier unless you count being able to subscribe X months in advance.