The world map illustration shows him as a proper Asian dragon - although it's possible his 'normal' form could be a seahorse?
(though... isn't the Japanese word "tatsunoko"? Like has already been used for the sea-serpent type enemies?)
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I wouldn't be surprised if Seiryu turned out to be the Auspice of a lizard or snake. That would give you a more Eastern looking appearance while keeping the dragon race as we know it separate from the Auspices.
With the story arc now finished, I think the most compelling thing about all four of them is how they all took on a more humanoid form when transformed, Suzaku being the most human-like of the four. I wonder why these animals took on such a distinctly anthropomorphic shift with the Echo.
Probably to be closer to and more like Tenzen (especially Suzaku.)