Well, the Kalos trio has other arguable origins besides the widely agreed-upon fan interpretation of Norse creatures:
Xerneas is a shishigami
Yveltal is an artistic license of the Yatagarasu, and may also be based on the Onmoraki.
Zygarde is an interesting case. There aren't any dragons, snakes, worms, or otherwise in all the monstrs and yokai of japan that could be a direct basis for his design. First thing I think of when I see him is a polar opposite to Rayquaza.
They're both green dragons that reside in a specific boundary (the lithosphere for Zygarde and the Mesosphere for Rayquaza) who are the mediator between a red Pokemon that has some relation to the end of life (Groudon, with his new Primal form and Desolate Land that hints toward being lethal enough to kill off living beings, and Yveltal, the Destruction Pokemon, both of which are also tied in to a unique boundary, being the Asthenosphere and the Stratosphere, respectively) and a blue Pokemon that's tied in to the creation of life (Kyogre, whose Primordial Sea ability alludes to the theory that life began in the ocean, and Xerneas, the Life Pokemon, who again are assigned a boundary, being the hydrosphere for the former and (arguably) the Troposphere for the latter).
-On a side note, I'm beginning to think they imbued the life/death theme into Groudon and Kyogre and then fleshed out the XY trio so they would have that parallelism between the two teams.-
Since Zygarde lives underground, it figures that it would look like something that percieves its surroundings differently than something that flies constantly, which is why Zygarde has specialized eyes, no mouth, a flat body other than its torso, and other features that attribute to its cave life. Rayquaza, on the other hand, perceives light differently, feeds on water molecules, has a cylindrical body and fins conducive to something that would require aerodynamics to function, etc. I'm also the fan of those who have made the comparison between Zygarde and a futuristic CPU, as something that sits and processes information from ecosystems all over the world, with flashing panels all over its body as the first indicator that it's working.
Also, they've both been given a prominent role in the mysteries of Mega Evolution, so that's something.
That was just me rambling. What bugs me is:
Yeah, I noticedthat Dunsparce is the same kind of critter. But there's no denying these things are the same kind of creature. They even resemble the myth more closely than Dunsparce does. Since they are actually children of the earth guardian. It may be that Zygarde is just a collection of all these little guys, as some are suggesting. Like a colony or hive-mind.
The legend of a Tsuchinoko is that it's a flat snake who can speak, enjoys drinking, will eat it's tail and roll along the ground like a wheel, and can jump up to three feet high. Dunsparce can do one of those, the Zooid has failed to prove itself in any of those specialties, as so far we've only seen it derp around and look very cute.
Also, "earth child" is a very distorted artistic license of the transliteration of "Tsuchinoko", which means "hammer spawn". I guess you could argue for that translation being poetic reason enough to tie it into the Nidhogg theory, since the hammer is widely recognized as a symbol of someone in Norse mythos...