Bosses' are unique types of Killers in Midnight Horrors, only appearing in special Boss Rounds, or appearing on special event maps. All Bosses so far have the ability to teleport players to a unique map for the fights to take place in. Each Boss has different unique mechanics, some Bosses have other gameplay segments before the main fight, some Bosses even have the players directly fight the Boss by gaining some kind of weapon to fight back with (Pharaoh King Solaris being a good example of this).
Minibosses have a looser description, but generally they serve as strong enemies that are fought before the much tougher main bosses. Minibosses rarely give event items or badges.
Bosses are periodically active and inactive in the game, and there isn't always a Boss that can be encountered in the current version of the game as they are usually reserved for special events/occasions. There are currently no active bosses.