Monster-of-the-day
Monster-of-the-day (or MOTD) refers to any of the expendable villains which usually showed up only for one episode. They were minions of more important villains, and were not valuable in the least. A monster-of-the-day is one of the most typical features in Japanese tokusatsu series.
They appeared in the anime and live-action series of Sailor Moon, as well as various Sailor Moon video games.
