Where are the rare spawns at?? (like Golden Poo Snake and so on)

The Golden Poo snake can be found on "Southern Deserted Island". It's in the southern most part of your map, in the middle.

As mentioned before, Golden Poo snake is on a triangle-shaped island in the south. To kill it, you MUST get the first attack (either have a thief equipped with the skill that increases the chance of first attacks, or equip pegasus boots), otherwise it will flee before you can do anything.

Equipping a dog collar and poo bracelet is recommended, too.

To clarify the above, the item to equip is called the Shoes of Hermes accessory, and will always guarantee your character the first attack in a battle, except for the case that the enemy gets a preemptive strike (which rarely happens). You can get this item from the Nothing Man, or find one in the Ancient Factory behind the black wall barrier.

The other "rare spawns" achievement-wise are the Gold Mecha Robo and the King Poo Snake. You can only find these monsters after you get all of the Mystery Parts and take them to the Mad Chief Researcher in Jibral Castle - Basment. You don't need to get both items, but only getting one will only let one of the enemies appear (can't remember which item spawns which monster). Gold Mecha Robo will spawn in the Ancient Factory, in the area after you ride the conveyor belt. He takes the place of a lone "red barrier" robot. King Poo spawns in the Ancient Ruins Cavern, after you beat the Jumbo Poo and the Corrosive Poo.

The only other monster that is kind of a rare spawn is the Platinum Poo Snake, which randomly appears on the Southwestern Deserted Island, where you find nothing but various Poo Snake spawns. The Platinum Poo Snake will pop up eventually.

There are other rare monsters that only appear by another monster summoning them in battle. Check the achievement faqs for them.