Because Au Ra aren't dragons? They just look a little bit like them.
They're also not beastmen, even as the line between "men" and "beastmen" becomes increasingly blurred. It sounds like only animals can become auspices, and Kojin are included, but there's no mention of it happening to the "human" races.
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It also occurred to me that Seiryu might not be a snake but a peiste, or a hydra if they count as normal animals. They're designed as snakes with legs to begin with, so it makes sense, and the model for Seiryu probably is reusing that 'skeleton'.