The Golden Poo Snake may be found once you've obtained the Mechat. Defeating the foe requires having the Shoes of Hermes and Poo Bracelet. He can be found on the Southeastern Desert Island along with a treasure chest. It's recommended that you have each of your characters in your party eat garlic or obtain the "stink" debuff as this will assist in the Golden Poo Snake pursuing your party on the battle field.
The location of the Golden Poo Snake is famous due to the amount of experience and SP that is awarded, if you are on a quest to obtain level 99 a majority of players will recommend using Field Barrier level 3 and pursuing the Golden Poo Snakes that spawn, assuming you have the Black Belt to expedite the amount of SP gained this is one of the quickest methods for reaching maximum level.