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All evolutions have either a type advantage or type disadvantage against the one on the opposite side.
Based on this and the assumption that Game Freak won't repeat a type, Sylveon HAS to be either Ground or Dark.
And before you say it could be a new type, consider this: this circle was given as a hint and clearly was meant to draw attention to type matchups. Why would Game Freak use this as a hint and then say "lolnope fuck you new type"?
Finally, does Sylveon look even the least bit like a ground-type?
Flying. FACT.
