I think he may have been a legendary super saiyan like Broly who became a Golden Oozaru and failed to become a super saiyan 4. But that's just my theory.
Most people think the LSSJ Broly was the same transformation as the SSJ mentioned by Vegeta in that past. Personally I think the original SSJ Goku was the same as the old SSJ mentioned by Vegeta because Toriyama had originally meant for the manga to end at the Frieza saga.














