Boos in Super Mario 64 were based on the angry wife of an assistant director of the game!
Super Mario 64's director Shigeru Miyamoto wanted Super Mario to be more detailed than previous games. He decided that some details should be based on real life. One such detail was inspired by the wife of assistant director Takashi Tezuka.
As Miyamoto described it, Tezuka's wife was usually very quiet. However, one day she just exploded at Tezuka--she was angry at him for all the time he spent at work. This inspired game designer to create the Boo character.
Boo shrinks when Mario looks at it, but when he turns away, it grows large and menacing. Passive aggressive much?