So it turns out that the Allagan Eureka we saw in the 6.3 live letter isn't from nowhere! A fair few people knew this already, and it's actually explained by an NPC in-game. But since nobody had copied down this NPC's dialog out-of-game as far as I could find, I've decided to get it myself.
All of this is from the Expedition Scholar in Eureka. The Scholar's actually hanging around the home base of every Eureka zone, but mostly only talks about the aetheric wackiness of the island, or to sell you Aetheric Stabilizers in Hydatos. But in Pyros (the third zone) she's actually available to talk to you about the original Eureka, the one that our Eureka was named after. I've copied it all down here:
Originally Posted by The ancient legend of Eureka
Originally Posted by The location of Eureka
I love that this actually has nods to both the FFIII Eureka, and the original plan for FFXIV's Eureka. For those not in the know, in the original FFIII Eureka was a hidden bonus dungeon, which contained both a bunch of bosses and the game's strongest jobs, Sage and Ninja. If you want to check it out now, it is in the pixel remaster, but isn't in the 3D version.Originally Posted by The treasures of Eureka
We've learned over time that there was a plan for a Eureka similar to that original in FFXIV; it was apparently what Nero's wrist communicator/detector thing blipping at the end of the Crystal Tower was originally supposed to be a hook for. We don't know what sort of content this original Eureka would've been, just that it was going to be located under the Crystal Tower.
And, for what it's worth: the name 'Orthos' was one of the alternative spellings of what's now commonly called Orthros, a two-headed dog from Greek and Roman myth; child of Echidna and Typhon and brother of Cerberus, it was killed as part of the Labors of Hercules. Orthros has never faithfully appeared in Final Fantasy, but the name was likely-mistakenly given to Ultros in a few early translations. Not sure how much any of that could matter--FFXIV very rarely does faithful mythical adaptations--but I do notice that with the exception of Hercules, every character I mentioned in this paragraph appears in FFXIV as a voidsent. A pattern of importance? Who knows, at this point.