The Theory doesn't make sense cause it's improperly implied. France, or well Gaul as it was known to RL Romans and such atleast 2000 years ago, was a SHITSTORM of Syncretism. You had Celts, GrecoRoman and what became "Norse" myths mixed together. The Yddgrasil Theory is bullshit desu. If anything Xerneas, Yveltal and Zygarde are an awkward mix of Greek/Roman And Norse Gods and Monsters blended together sprinkled with Hindu and Celtic tones. Basically Indo-European Myths found across its descended Cultures that would fit the time scale of 3,000 years ago.
Xerneas: Based on Artermis/Diana given the Deer (specifically the Cerynean Hind) tied to life-giving properties, Brahmaa given its role as creator and the connection to the number four (X having four points), the Horned God Cernunnos and the Arthurian Merlin (as Merlin was tied to wildlife, deers and becoming a magical tree in some stories)
Yveltal: Based on Apollo given the Crow-esque features and being a destroyer (perhaps through the Coronis myth), Shiva given its role as destroyer and the number three (Y has three points), the Morrigan a triple-crow goddess of War and Death (Sounds familiar eh?)
Zygarde: Based on the myth of the original Oracle of Delphi, an embodiment of the earth in the form of a big ass snake named Pythia, Vishnu given their associated with the number five (Z has "five" points I guess?) and snakes (legit has a five-headed snake for a mount,) along with the two's shared role of Preserver. The Norse myth is heaviest here since you have Fenrir the Dog (or more so Garmr a supposed Underworld variant under Hel's control), the Midgard Serpent which makes a lot of sense since Zygarde is legit a "Serpent" who can wrap itself around the world (Via its Cores and Cells) and lastly Perfect form is a lose, genderless variant of Hel (representing the powers of Life and Death but with none of the Undead Waifu features)
