So far, we know of three bosses. They are:
The Ghost Toad Librarian (mini boss):
The Giant spider (main boss in one mansion):
And King Boo:
But what do you think the other bosses will be? After all, that's only the final boss and the Gloomy Mansion bosses, what are the bosses going to be in the Old Clockworks, Abandoned Laboratory and plant themed mansion?