Jim Crow
Jim Crow is a theater character that was developed by Thomas D. Rice, who would perform in blackface as the character. The stage persona eventually led its name to negative stereotypes of black people. Later, “Jim Crow” segregation laws came into effect until Lyndon Johnson signed the Civil Rights act. Jim Crow was never an actual person.
John Doe and Jane Doe
John Doe is the multiple-use name given to a male person whose true name is unknown or is being concealed. Jane Doe is used for females. These names don’t refer to a historical person named John Doe but was rather chosen as it satisfied the technical requirements of jurisdiction. There are folk etymologies for the name but nobody knows the rationale behind the names.