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Monster-of-the-week is a fan-created term  that refers to any expendable minion who appear in one (or, in some rare cases, two) episodes, serving under one of the more powerful villains to complete some nefarious objective.
Almost always, they are "healed" or die through one of Sailor Moon's signature attacks (Moon Tiara Action, Moon Princess Halation, Moon Spiral Heart Attack, Rainbow Moon Heart Ache, Moon Gorgeous Meditation) with the exception being Regulus or by one of the Sailor Senshi (e.g. Sailor Mars fatally scorches the youma Kigaan as well as two of the Doom and Gloom Girls).
They appear in the anime, manga and live-action series of Sailor Moon "as well as various Sailor Moon video games". The only ones that only appear in the anime are phages. The following monsters-of-the-week exist in the Sailor Moon series:
- Youma - Also known as a Negaverse monster in the dubbed American version of the anime. The Youma in the manga, like the ones in the anime, disguise themselves as humans under Jadeite's command until they attack.
- Cardian - Kept inside cards working for Ail and An to drain energy for the Makaiju. The Cardians are exclusice to the anime and absent in the manga
- Droid - sent by the Spectre Sisters (Koan, Berthier, Calaveras, and Petz, who were working for Rubeus), to take over crystal points, and later Esmeraude, to fill Tokyo's vulnerable spots with dark energy. One droid, Jakoku, worked for Prince Demande. In the manga, almost all Droids look exactly the same, and assisted the Spectre Sisters in their missions
- Daimon - Who extracted pure hearts from humans under the orders of Kaorinite, Eugeal, and Mimet in Sailor Moon S. In the manga, Daimons possessed humans rather than inanimate objects, and they were monstrous forms. The Witches 5 were considered perfect Daimons in the manga.
- Lemures - Who worked for the Amazon Trio and Amazon Quartet in order to defeat the Sailor Senshi whenever they tried to foil their plans as seen in Sailor Moon SuperS. In the manga, Lemures were small formless shadow creatures who attacked in large groups.
- Phage - A normal human whos Star seeds has been taken out and corrupted by one of the Sailor Animates who work for Shadow Galactica. They are often parody of a Sailor Senshi and their appearances vary depending on their personality, trait or occupation. They only appear in the anime. It should be noted that in the manga, a normal person does not turn into a phage after their starseed has been taken out.