What is a word that describes something that (potentially) never existed?
You can use "alleged" to mean "claimed but not proven (to exist)".
"John claimed that the alleged journal was 'lost'."
ODO:
alleged ADJECTIVE
[attributive] Said, without proof, to have taken place or to have a specified illegal or undesirable quality:
‘the alleged conspirators’‘The British Museum, too, says it knows nothing about any alleged fakes in its displays.’
- Imaginary: Existing only in the imagination or fancy
Example: He blames the hold-up on an imaginary broker he calls
- Illusory:Causing illusion; misleading
Example:In the end, though, the size of the field staff, the number of offices, and the number of early voters might be illusory.
- Supposed: Generally assumed or believed to be the case, but not necessarily so.
Example: Vietnam is the supposed site of an ancient temple
- Hypothetical: Imagined or suggested.
Example: This is all very hypothetical but supposing Jackie got the job, how would that affect you?