Hwangwoong is dressed as Hwanung, a God in Korean mythology he was named after. Leedo, Xion, RavN, Seoho and Keonhee are dressed as bears and tigers.
Hwanung ("Supreme Divine Regent"), the son of Hwanin ("Lord of Heaven") wanted to live on Earth. His father agreed to let him leave Heaven and go on Earth with some of his followers. He founded Sinsi ("City of God") and met a bear and a tiger near the woods. Both animals lived in a cave but pray each day to Hwanung. The God promised them that he would turn them into humans if they stayed in their cave for 100 days and only ate the garlic and mugwort he gave them. The tiger grew impatient pretty quickly and left the cave, leaving the bear all alone. After 21 days, the bear turned into a beautiful woman. With time, she became the wife of Hwanung and gave birth to a son, Dangun.
Source: ONEUS YouTube