Au Ra, what are they?

[Atoli]

I wouldn't say that. Being a Warrior of Light is about being chosen by the planet itself, which means she can probably make up the rules as she chooses. A Garlean WoL could happen just as soon as she chooses to make it happen.

I think we don't even know why Garleans are unable to use magic (something to do with that third eye), so declaring that Hydaelyn can just make up the rules as she pleases is probably going a bit far.
We've seen enough things that were way beyond her control already, and that's not even counting the calamities happening. She couldn't even protect her chosen WoL from Midgardsomr's interference.
All you'd need is an equally strong being behind the Garleans inability to weave aether and Hydaelyn couldn't do anything at all.

@thread topic:
It's all been said already (that they are meant to be demonic looking), but I still like to imagine that they are a dragon x human lovechild (don't worry about the logistics of that, they reproduce asexually...which probably still holds true for Au Ra to this day, since that size difference is nothing to laugh at either^^°)