A Mini-Boss is a type of boss in the Metroid series 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 the final boss, Mother Brain. For their later appearances, they were considered regular bosses since hardware improvements led to an increase in their size and power.
In the manual of Super Metroid, the Spore Spawn and Crocomire are officially designated as the Mini-Bosses of Brinstar and Norfair, respectively. Spore Spawn's battle music is titled Mini Boss Confrontation BGM and is shared with Botwoon, which is also referred to as a mini-boss by Super Metroid Nintendo Player's Guide.
In Metroid: Zero Mission, the Imago is referred to as a miniboss by Metroid: Zero Mission: The Official Nintendo Player's Guide.
Otherwise, it is not always clear where the distinction lies between stronger enemies and Mini-Bosses, or Mini-Bosses and weaker bosses; this distinction may be up to the player's discretion. 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. In these cases, the Mini-Bosses have a special music theme, such as VS. Giant Beetle, during their first encounter that is not shared with normal enemy encounters.