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Sylveon is a fairy type
1. Its names through each language reference fairies:
a. Sylveon -> Silvia -> Spirit of the wood -> Fairy
b. Nymph -> a deity of the forest -> Fairy
c. Feelinara -> Fee -> German for Fairy
2. XY have a biology theme. A new type fits in with the theme.
3. Sylveon looks like none of the existing types except for Normal. Other Normal types, such as Clefairy and Snubbull, represent fairies. There is also Meloetta, who is fairy-like in design.
4. Probably bug because it isn't an anthropod like pretty much every other bug type. Not pure flying because it hasn't been seen flying. Also, for flying type, its name through each language don't relate to flying type. How does Ninfia reference flying? How does Feelinara reference flying?
5. Might not mean much, but the Japanese site has Sylveon's type in a pink type box, different from all the other type boxes.