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An onomatopoeic fox noise, probably a growl.
It's somewhat common for fox characters to say something to this effect on occasion, and the word "konkon" (コンコン) is even a diminutive word for foxes, but perhaps no one uses it more iconically than Touhou Project's Kudamaki Tsukasa, for whom it can be considered a catchphrase. This stems from the fact that one of her in-game portraits displays her saying こーん right before her battle, notably written in Hiragana rather than Katakana, the alphabet typically reserved for onomatopoeia. Many fan works show her saying this, and Akatsuki Records even made a music video based entirely around Tsukasa making ko~n puns named "Ko~nsultant!."
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