Fan speculation on our forums is that the three new formes are inspired by Aztec legend.
Tornadus is Huitzilopochtli, who after it died became a green and purple hummingbird,
Thundurus is Quetzalcoatl, who after it died became a serpent, and
Landorus is Tezcatlipoca / Tepeyollotl, a jaguar. In the Pokemon AR Searcher screenshot on the left side of the 3rd scan and in the anime poster on the right side of the 7th scan, you can see each of them resembling those animals (a bird, a snake, and a jaguar).
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