On another topic, Mettaton EX's battle theme,
Death By Glamour, is actually a mashup of a couple different music pieces that relate to him. The opening of the song is a more poppy remix of
CORE, then it transitions into a remix of his original battle theme,
Metal Crusher, and the end is a more enthusiastic rendition of
It's Showtime!. Turns out this is actually a common trend with Undertale's music, and a lot of boss battles and themes for important plot points sample tracks found elsewhere in the game. This is actually a really interesting way to get by without making a ton of unique tracks, and capitalize on the significance of each music piece by subtly implementing them repeatedly throughout the game. Some other interesting examples of this are hidden below due to spoilers: