Board Thread:General Discussion/@comment-4504826-20160516040951/@comment-5322753-20160815140119

Arthurtilly wrote: [...] that they don't have their own theme and instead use the "Enemy Approaching" song.

They do have their own song if you polish the armor of 02 and then whisper to 01. They are definitely a miniboss with Muffet being the Hotlands boss and Mettaton (or Mettaton EX if you want) being the CORE boss.