Boos

Boos, also called Teresas in Japan, are a group of cute ghosts that are regular enemies of Mario who usually haunt dark castles. Due to the fact they are extremely shy, they freeze up and cower in fear whenever anyone looks directly at them, but when someone's vision is strayed away from them, they'll attack.

They first appeared in Super Mario Bros. 3.

Appearance
All boos are portrayed as white translucent spheres with matching stubby arms, a "tail", and mischievous facial expression.

Trivia

 * Their names are a reference to the exclamation "boo!", a term relating to surprise or fear.
 * Their original names, Boo Diddleys, is a reference to Bo Diddley, a famous musician.