As far as I'm aware Honeygast, Mitsuro and Mirrim are the only bosses in the game. Honeygast being the only full on boss while Mitsuro and Mirrim being mini bosses.
Mirrim can be found in the Observatory, Funhouse and the Dilapidated Halls. Though in both she has a 1 in 256 chance to spawn. So be ready to face her. Idk if she can be fed or eaten though since she's the only one I can't play with in the Ghostiary yet despite eating and feeding every other ghost there