Actually they both originally shared the exact same name - セフィロス(Sephiroth/Sefirosu).
The Fiend's name is never really mentioned in FFVI though and is only known from the concept sketch, same with the other Triads. It is possible that due to that devs simply decided to reuse the name for the next game's villain.
The spelling change in FFXIV was probably done to avoid confusion with the more popular, but not original, bearer of the name.