Tequatl and Blightghost
They certainly use the same model. It seems to be a form Zhaitan likes using for his draconic champions. There are a few of them in the skies over Orr.
All the Orders eventually lead to the battle against Zhaitan in Orr.
All of the dragon champions of Zhaitan seem to utilize the same model. The three fought in the Arah storymode dungeon, as well as the numerous ones throughout Orr and the Ara story dungeon all have the same model as Tequatl and Blightghost.
The Orders’ storylines merge/end at lvl 60, when you kill Blightghost – after that, it’s the Pact’s storyline which divides at two parts: what your fear is, and the one of three plans chosen for invading further into Orr (specifically, invading the Cursed Shore).
Stop treating GW2 as a single story. Each Season and expansion should be their own story.
The real interesting question is, what were they originally; it appears there was some native Tyrian species of dragon (see: Rotscale) that has long-since been driven extinct… but hardly anything is known about them, beyond the fact that they seem to have the propensity for being resurrected time and time again.
Personally, I suspect the Bone Dragons were tied to Zhaitan. Those “heads” that was Zhaitan’s mouth looked very similar to Bone Dragon heads. Furthermore, just like Bone Dragons, Zhaitan lacks a lower torso – it’s just tails and more wings. No hind legs.
Stop treating GW2 as a single story. Each Season and expansion should be their own story.