Board Thread:General Discussion/@comment-32789293-20171015183110

Every boss or mini boss in Undertale has a theme song. (except RG 1 and 2 and almost all of the canine unit) Maybe the song that plays when you fight is the song of the most powerful in the battle. This makes some sense as, example, when you fight the CORE enemies in the Ruins (AKA Hard Mode) thier theme plays but when you fight when Frisk is more experienced your theme plays. 