I guess if Biggeron is the deity, then he should be the ancestor of Valoo (like the other OoT deities)...
So... A large goron is the ancestor of a dragon :S

OR...

Volvagia, a large dragon is the ancestor of Valoo, a large dragon...

Tough decision...

Also, a deity doesn't need to be good. There are plenty of evil deities that cultures all around the world believe in even today...

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