But the thing is, the serpent dragon does exist in the data. There is no need to "confirm its existence" if it's there lol. It's prolly just stuff that's supposed to be hidden though, similiar to the slime kitty this season, or the dark ranger stuff.
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Even the recent bluepost said " throughout your leveling experience, you’ll earn four different Dragon Isles Drakes". That doesn't mean we won't get any further dragonriding mounts via max level content. Actually, the gladiator mount is supposed to be usable for dragonriding aswell so that's already another confirmation that we will get more than these 4. And again, the serpent mount with the same amount of customization as the other ones currently have was already datamined. It exists, it looks good, it's not a placeholder, and we will get it. It's just something that prolly was supposed to be encrypted.