Zardnaar
Legend
That Dragon isn't unique. There is only one that has fully transformed, but every Sorcerer-King is somewhere along the process of transforming into another one.
Dragonborn look a lot less like the dragon than a lizard. People wouldn't make that connection. The transformed dragon isn't in any way humanoid or color coded like Dragonborn are.
Colour coded dragonborn shouldn't exist on Athas. Maybe as mutants.
Other Dragons exist. That knowledge isnt public.
You've never read the story "A Little Knowledge"? It basically defines the settings major themes.
Its why WotC keeps Spelljamming things up.