Second or other self or alternative self
Cicero coined the term as a part of his psychological construct in 1st Century Rome.
Freud considered the roots of the phenomenon of the alter ego to be in the narcissistic stage of early childhood.
In Literature
Robert Luis stevenson’s thriller Strange Case of Dr Jekyll and Mr Hyde
In the novel The Count of Monte cristo by Alexander Dumas, the main chracter Edmund Dantes
Norman Douglas in 1890s short story The Familiar Spirit.
Fight Club by Tyler Durden
Yellow Wallpaper