Please only add bosses once you have confirmed what floor they are on (hold TAB in-game if you are uncertain). Do NOT assume bosses appear in the same areas as they did in the original Isaac. Unconfirmed bosses have asterisks around their name.
Bosses are strong monsters found in boss rooms at the end of a floor. The boss at the end of the floor must be defeated in order to advance to the next floor. In later chapters, bosses are able to spawn as normal enemies. The boss for a particular floor is chosen from the pool of bosses for the environment of the floor. Bosses may also spawn as champions, which can change various attributes such as health, damage, and attack pattern. Upon defeat, most bosses will drop an item from the boss item pool. Additionally, some floors may spawn a room with a Mini-Boss.
Mini-bosses can be encountered on any floor, and will appear on post-Womb/Utero floors as regular enemies. Unlike with regular bosses, there is no special door leading to a mini-boss room, meaning that there is no way to tell whether a room is inhabited with a mini-boss unless the player is in possession of an item that reveals map icons. Once in a mini-boss room, the player cannot escape through means of bombing the door, as it will be boarded off.
Each of these mini-bosses has specific drops, that are consumables or a pedestal item (less frequently).
In a room containing multiple mini-bosses, only the last one will drop something, allowing you to choose what drop you would want by killing a certain mini-boss last.