Wan comes from the ancestors of the Fire Nation (presumably), and while the Avatar world in general is heavily influenced by Asian culture, the Fire Nation most resembles Japan.
So it makes sense, if you think of it, that Wan encountered spirits that behaved the way that the spirits of Japanese religion and myth have also behaved in the past. Ancient Japanese religion and that particular take on the spirits also influenced Miyazaki's works, which is understandable because, you know, Miyazaki was Japanese and all.
But it is also no big secret at all that both Avatar series were heavily influenced by anime. So much so that people who don't know what the origins of the shows are assume they are anime.
None of this is very surprising... Surprisingly, o________O
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