doe john

from The Collaborative International Dictionary of English v.0.48

Doe John \Doe, John\ (Law)
   The fictitious lessee acting as plaintiff in the common-law
   action of ejectment, the fictitious defendant being usually
   denominated {Richard Roe}. Hence, a fictitious name for a
   party, real or fictitious, to any action or proceeding.
   [Webster 1913 Suppl.]