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Literally my moms 10% would be based on Fenrir's offspring, Sköll and Hati (wolves which persecute Sol, sun god, and Mani, moon godness) not only one single wolf just like the anime shows two different "incarnations" of Zygarde (Z1 and Z2)
Same about 50% forme being based not only on Jörmungandr but Nidhögg too, since both are giant serpent creatures.
100% is based on Hel/Hela godness, supposed to be "half live and half dead", half beautiful and half horrible, she represents how reach the perfection combining opposite forces (that's why Zygarde's wings represent Xerneas and Yveltal's colors). "HeLa" casually is the name of a cell type in an immortal cell line used in scientific research, the oldest and most commonly used human cell line.
Plus, Zygarde's rainbow chest is based on Bifröst, the bridge between Midgard and Asgard. Since Xerneas represents Midgar's deer and Yveltal Asgard's eagle, Zygarde's rainbow is supposed to be the bridge between life and death (its chest also looks like the french flag when is closed, but is clearly a rainbow when is open, and the rainbow also represents diversity, the whole and complete color spectrum, and rainbow is also a hawaian/alolan national symbol as we can see in SM).
