The hypothetical state of Hwanguk (환국, 桓國) is the first mythical state of Korea described in Hwandan Gogi. According to Hwandan Gogi, Hwanguk existed long before Gojoseon. However, mainstream Korean historians reject the existence of Hwanguk for lack of credible evidence.
Claim of Hwanguk
In some editions of Samguk Yusa, one of the oldest surviving Korean history books, the name Hwanin (桓因), the mythical grandfather of Gojoseon's founder Dangun, appears as if it is instead written Hwanguk (桓國), which would mean "The Hwan nation." Some people argue that this is an evidence that the original manuscript of Samguk Yusa wrote Hwanguk instead of Hwanin.
References
Hwan-guk Wikipedia(Text) CC BY-SA