Ploni Almoni is known by a number of names around the world. He is:
and the list goes on.
Basically, there are any number of uses for a generic person, from the formal (unidentified bodies) to the business world (the "Man in the Street").
Ploni is Hebrew's John Doe. In this it's a little unusual. The word for an unidentified person is normally country-specific, not language-specific. The UK and USA use different names, as do Canada and France.
Hebrew is a little different because the use of name Ploni dates back to the Bible.
Then Boaz went to the gate and sat there, and just then the redeemer (that Boaz was speaking about earlier) passed, and he said "come, sit here Ploni Almoni", and he came and sat.
This 'redeemer' is a close relative of Ruth's dead husband. As such, he should have redeemed Ruth (that is, married her), but he didn't want to (Boaz himself did in the end). While Ploni Almoni might be a real name, it is usually interpreted as a pseudonym, put in the book either to save the man embarrasment for not fulfilling his duty, or as a punishment to him. Even the King James Version translates "Ploni Almoni" as effectively "Oi, you".
This usage for Ploni found its way into the Talmud, where it was used in Jewish Law. It's used today in prayer books, where "Ploni son of Ploni" replaces Insert Name Here. Plonit is the female form.