In the Great Ancient Ruins, there are uncommon enemies called Moneys that sometimes steal your gold. Let them be for the meantime.

After clearing the dungeon, return to Terrace Cave and find some sparkling stuff on the floor. Pick them up (each is worth 1G), as it leads the trail to the NE. Find a chest after the last coin. Open it to enter a battle with a secret boss named Money-Money. Defeating it will reward you with another hint-book for the Backyard Battles in Gohtza. You will also recover all your stolen money back.