A Dragon from the Dragonborn DLC spawned but I don't have the DLC
This is more than likely a bug
As with most game developers, Bethesda factored some upcoming DLC changes into their standard game updates. As such it's likely the data for this dragon is stored within the game installation on your Xbox thanks to an update, but a bug has allowed this hidden content to be pulled live into the game.
That's the only reasonable explanation I can think of for DLC exclusive content to appear within a vanilla game.