Freud's "return of the repressed"

The term 'return of the repressed' was coined by Sigmund Freud. The return of the repressed is the process whereby repressed elements, preserved in the unconscious, tend to reappear, in consciousness or in behavior, in the shape of secondary and more or less unrecognizable "derivatives of the unconscious."

In other words, underlying or 'repressed' emotions and behavior may emerge and show on the surface, in subtle or perhaps even significant ways.

This is a base for many horror films, which most likely started with the novel Strange Case of Dr Jekyll and Mr Hyde written in 1886 by Robert Louis Stevenson. Some films that use the same idea of something evil hidden within, only to emerge, are Halloween (1978) and An American Werewolf in London (1981) as well as most slasher films.