I will get into the details a little later, but the Zodiac Stones are one of the larger connections to Ogre Battle, Matsuno's previous series for Quest which is a soft prequel to FFT, with it often being speculated Tactics plot was built on the bones of the never produced finale to that franchise (something Matsuno would do again with Crimson Shroud being built from Vagrant Story 2's drafts)
Within Ogre Battle, the Zodiac Stones were a gift from the gods which could only be used by the "worthy" (ie, someone with a high reputation score) for a massive power boost, having been previously been used by the Twelve Sages (basically the Zodiac Braves). Collecting all twelve would allow player character Destin to get the True Ending.