A Mini-Boss is a type of boss in the Metroid which, while still posing a challenge, is smaller and/or weaker than a regular boss in the respective game. Defeating a Mini-Boss is sometimes necessary to accomplish a larger goal or reach an item. In the original Metroid, Kraid and Ridley were called "Mini-Bosses" as their defeat was necessary to access Mother Brain. For their later appearances, they were considered to be regular bosses as hardware improvements led to an increase in their size and power.
Some Mini-Bosses are stronger versions of regular enemies, such as the Plated Beetle, Sheegoth and Cloaked Drone, and may appear later in the game as normal enemies. Certain battles with groups of Space Pirates can be considered Mini-Boss fights as well.