Personally, I do see the Kalos Trio being based off of Norse Mythology, but more generalized, and not drawing inspiration from just the one myth. Like, eagles in general are associated with death in Norse mythology. Not just Hraesvelgr, but other beings such as the God of Death (who turns into an eagle), and the Blood Eagle ritual. (Also, Zygarde is more Jormungand than Nidhogg, with its other two forms likely being based off Fenrir and Hel.) Those are just my thoughts: you’re free to disagree.
Let the disagreement commence. [rolls up sleeves, cracks knuckles]
It is honestly baffling to me that this idea is so widely and unquestioningly accepted, because personally, I don’t think I’ve ever been less convinced by a Pokémon fan theory in my life. I don’t even understand why people look at Yveltal and think “eagle.” The “ruff” around its neck is almost certainly meant to make us think “vulture,” which is a much easier association with death. Stags can be associated with nature without having to bring Norse mythology into it; birds of prey or carrion birds can be associated with death without having to bring Norse mythology into it; insisting that Norse mythology has anything to do with these Pokémon makes the concept weaker and more confusing.
I can’t find any reference to a Norse god of death who takes on the form of an eagle, and I admit Norse mythology is not my field, but I still feel like I’m justified in saying that this is obscure enough to be a really weird choice. They’re associated with death in Norse culture to the extent that they’re known as carrion birds – they pick at corpses on the battlefield – but that association isn’t unique to either the Norse or eagles; that exists all over the world and with birds of prey generally (sky burials in central Asia, slain warriors becoming “food for the birds” in the Homeric epics, “Thunderbirds” in pre-Columbian America that prey on humans, the role of Aztec eagle warriors in procuring human sacrifices, wall paintings from Çatalhöyük that show vultures feeding on headless corpses, etc.). And I’m trying to skim as much as I can find on JSTOR about blood eagle sacrifices and as far as I can tell no one even knows whether they were ever a real element of Norse culture; the practice is supposedly 9th century, but our first surviving reference to it is a 12th century quotation of an 11th century source. The original Old Norse is difficult to translate and sparse in detail, and the fuller accounts of later periods are touched by a desire to make Norse paganism seem weird and frightening for a Christian audience. The whole blasted thing comes originally from a line of Sighvatr about “carving Ælla’s back like an eagle,” which is more naturally taken as “the way an eagle would,” rather than “in a gruesome ritualistic depiction of an eagle” (Roberta Frank’s article in the 1984 issue of the English Historical Review seems to be the classic takedown of the whole thing). I’d be inclined to put most of it down to Christian sensationalism.
That part wasn’t important but I am here to educate people, damn it!
And there are so many choices Game Freak made in designing these Pokémon that just make no sense if they were trying to evoke Norse myth. Why would they make Zygarde an underground creature if they meant it to reference a sea monster? Why make it a guardian of order when all of the mythical entities they were trying to reference, along with the parent those entities share, are harbingers of chaos? Why base one of its forms on the goddess of death when a different member of its trio is supposed to represent death? If they were committed to the World Tree as a unifying motif, why use a white tree for Xerneas’ dormant form and then a weird cocoon thing for Yveltal’s? Why concoct for Zygarde’s transformations the mechanic of cells and cores, which have no resonance whatsoever with its supposed mythological inspirations? Why, if they wanted to use Norse mythology and wanted to make letters of the alphabet (X, Y, Z, AZ) an important motif, would they decide not to even touch the runic alphabet? Why ground a story about the atrocities of war in the mythology of a culture that grants its finest version of the afterlife to people who die in battle? Why create legendary Pokémon who symbolise the harmonious equilibrium of life and death, which Lysandre threatens to disrupt by bringing about an apocalypse, and base them on a mythology in which the apocalypse of Ragnarök is an inevitable part of the cosmic order? Why, for goodness’ sake, do all of this in a region based on France, in a plot that references the French Revolution?
And… there’s not really any link between any of these beings that would make them a sensible choice as the inspiration for a trio of legendary Pokémon. They don’t go together in any meaningful way. Like, none of these eagle beings have anything to do with the mythical stags that people cite for Xerneas, or with Jormungandr. If you’re okay with Zygarde being Nidhoggr then it makes sense for Yveltal to be the eagle that sits at the top of the World Tree, but then you have to explain why the hell Ratatoskr the squirrel is missing, because he is kind of central to their relationship, and Xerneas becomes kind of a third wheel.
If these are the influences the designers had in mind, then they executed the concept terribly. They did almost nothing with the source material, and what they did do makes no sense. I think it’s much more charitable to assume they had no such intention.