This would be the post in which someone posted that he saw it spawn:
None of my “hunts” to see if they actually exist ingame has borne any fruit though, so I can’t confirm anything sadly.
Edit: But yes, I’m starting to think you’re right and that it isn’t ingame. Or it’s an insanely rare spawn at least. Becauuse as you say, the loot it drops is connected to stuff not ingame yet, so it would make sense.