Ah, thanks for the details of the Japanese script. That's very handy and enlightening. While it's now clear that Griever can't have been a previously-existent GF, I'd say that there still must've been some old 'Griever legend', like Sir Bahamut says. This does, however, effectively debunk the old "R=U" support evidence which maintained that, since Ultimecia owns Squall's favourite GF, then there "must" be some kind of connection.

I speak German, so the German script would be interesting to see sometime

Edit: Unknowns, that's exactly what I've always believed! It's actually supported by dialogue in the game, too. Onboard the Ragnarok in disc four, Rinoa talks about how simply defeating Ultimecia won't change the fact that she'll still be born. She discusses how her own powers will be passed on through the generations, eventually reaching Ultimecia. "What does it mean to inherit Ultimecia's power?" she asks.