In the Crystal Tower raids, we got the Cloud of Darkness from FF3; she's narrative relevant, but not much of an actual character here in XIV.
In the Omega raids, we got Chaos (FF1), Exdeath (FF5), and Kefka (FF6) as single boss fights, with the context that they were not really relevant to our world (can't remember if they were extraplanar, fictional, or something else) and just recreations by Omega for us to fight.
Then in the Endwalker patch quests, we've got Golbez, who is very clearly from FF4 but has an MSQ role and new XIV-specific context and character.