What do you think is the oldest English-language story that mentions lycanthropy? I would guess that "The Duchess of Malfi,"
http://en.wikipedia.org/wiki/The_Duchess_of_Malfi, a John Webster play that was first performed at the Globe Theatre in London in 1614 might qualify.
Here's an excerpt from the part that describes lycanthropy:
Act V, scene ii
. . . . PESCARA: Pray thee, what's his disease?
DOCTOR: A very pestilent disease, my lord,
They call lycanthropia.
PESCARA: What's that?
I need a dictionary . . . .
DOCTOR: I'll tell you.
In those that are possess'd with't there o'erflows
Such melancholy humour, they imagine
Themselves to be transformed into wolves;
Steal forth to churchyards in the dead of night,
And dig dead bodies up: as two nights since
One met the Duke 'bout midnight in a lane
Behind St. Mark's Church, with the leg of a man
Upon his shoulder, and he howl'd fearfully;
Said he was a wolf, only the difference
Was, a wolf's skin was hairy on the outside,
His on the inside; bade them take their swords,
Rip up his flesh, and try: straight, I was sent for,
And having minister'd unto him, found his grace
Very well recover'd . . . .
See
http://larryavisbrown.homestead.com/fil ... lfi_Vb.htm