Rainbow stag beetle (likely)

Native to northeastern Australia, especially subtropical rainforest and woodland. It is not a naturally occurring Korean species.

Adults drink tree sap, nectar, and fruit juices; larvae consume decaying wood.

Mostly active from late spring through summer, often attracted to lights; males use enlarged mandibles in contests. The orange-and-black pattern and curved mandibles strongly match this species.

Harmless to people and plants; larvae aid decomposition. Because the image was recorded in 대한민국 on September 6, the identification conflicts with the species’ native range, so this is likely an escaped or imported beetle, or a visually similar exotic lucanid.