Board Thread:General Discussion/@comment-32432695-20170818134603/@comment-13682699-20170819220031

Actually, the ghosts do have SOULs. Its just their incorporeal nature allows them to avoid having their SOULs taken. Also, Mettaton has a normal SOUL just like everyone else, the only confirmed boss monsters are Toriel and Asgore. Monsters with boss-level encounters are not considered boss monsters in that respect.