Guess I'm not sure what you mean by optional dungeons, because in my book most of the PSX FFs had them. FFVIII at least had the Deep Sea Research Lab,which is certainly extensive enough to count in my books, and I would also say Centra Ruins is an optional dungeon.

FFVII had the Ancient Forest and Gelnika. Gelnika was maybe a bit small to call an optional dungeon, but the Ancient Forest we a bit bigger. I think it counts.

I can't really remember if FFIX had any, but kind of surprised if FFIX of all games didn't.