The Boos are recurring and iconic enemies of the Mario franchise.
It's a former character in the license since it first appeared in Super Mario Bros. 3 in 1988. Later, it is found in almost all games and even in other series derived from the universe of Mario, such as those of Yoshi Island or Luigi's Mansion. It will even make appearances in other Nintendo classics like Tetris DS or The Legend of Zelda: Link's Awakening.
Shy by nature, it has the particularity of becoming harmless when the player is in front of it: it hides its eyes, and only attacks when its prey is from behind. There are several species of Boos, but all of them have this trait.