Why does Guan Yin ride a dragon?

Someone had once asked here "Why is Kwan Yin riding a sea turtle?", but what I'm more curious about is why is she seen on a dragon? There are many images or the Bodhisattva riding a dragon, one I posted being but one example. But who/what is the dragon? Is there a story behind it? I've heard the dragon is a symbol of wisdom and purity, others have suggested the dragon represents a wrathful force subdued by Guan Yin underfoot, but really what's the case here? Is it merely symbolic or is the dragon a literal entity (sentient or otherwise)? Any information would be appreciated (not modern "New Age").