It's out of era, so if it's safe to assume the former is correct. I've not seen it myself either, and visiting the spawn spot right now it's not around (and unless it's different on nasomi for some reason, it should be a timed spawn).

The shield it drops, Pelte, has an entry in the AH database, but no recorded sale. I don't know whether the fact an entry exists means the item was sold at least once before.

The AH database on the nasomi website seems to include any item in the current version of the game client (from what I remember, the nas client is using a 2017 build of the game? Correct me if I'm wrong) which includes out of era and Lv 76+ items. (For example, searching "Aurore" brings up the lv 76 pieces of entry-level Abyssea gear).

No recorded sales is the best indicator for whether or not the item (and it's NM) are active on the server. (though there are a few instances of no recorded sales for in-era NMs, which could mean that no one's bothered farming them, or they're missing NMs too!)

In this case, it's probably best to remove Duke Decapod from the list of NMs until someone posts proof that it actually exists