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I actually feel like the Ultra Beasts are easy to work into any random region compared to most Legendaries (other than the birds). They're just invasive species and not individuals with specific lore; you just have to say that some successfully spread out from Alola or that someone else did some dimensional fuckery. Compare that to box Legends, Lake Guardians, or Tapus who make 0 sense appearing out of their original region.
