The OED has this to say about the origin in legal history of the standard names John Doe and Richard Roe:
John Doe, (a) (Eng. and U.S. Law), the name given to the fictitious lessee of the plaintiff, in the (now obsolete) mixed action of ejectment, the fictitious defendant being called Richard Roe; (b) name given to an ordinary or typical citizen (see also quot. 1942)
The earliest citation given is 1768:
1768 BLACKSTONE Comm. III. xviii. 274 The security here spoken of..is at present become a mere form: and *John Doe and Richard Roe are always returned as the standing pledges for this purpose.
Further enlightenment, including the distinction between personal chattels and chattels real as applied to fresh and unfresh fish, may or may not be available here, in a fascinating discussion which I have not had time to read.
Posted by Mark Liberman at July 30, 2004 01:53 PM