JOHN DOE

A person whose true identity is unknown or must be withheld in a legal action is called John Doe.

1. Origin of this use is certainly unknown but there are some suggestions that these names are used from medieval period probably beginning from the early reign of England's King Edward III (1312–1377) and especially used to invoke English Legal instrument.
2. Use of the names was followed by a British legal process called an Action of ejectment which was used in Old English law. According to it,the landowners couldnt take action against the squatters who occupied their lands. So they would took an action of ejectment (Ejectment is the common law term for civil action to recover the possession of or title to land) on behalf of a fictitious tenant against another fictitious person who had been expelled.
3. John Doe is used for men, Jane Roe for women, Johnnie Doe / Janie Doe / Baby Doe / Precious Doe for children and simply Doe for non gender specific. 
4. The names also refers to a corpse or hospital patient whose identity is unknown.Commonly used in urban areas where a hospital or officials who deals with large number of unknown persons.
5. These naming are just used only in USA and Canada and other countries like UK, Australia and New Zealand uses different name to refer the unidentified person.
6. They are especially used in US police investigations when the victim(s) identity is unknown or incorrect.
7. A Doe subpoena is an investigatory tool that a plaintiff may use to seek the identity of an unknown defendant.
   
8. Doe subpoenas are also often served on online service providers and ISPs to obtain the identity of the author of an anonymous post.
9. Courts also allow the names to be used by people who do not want to give their real names in certain cases.