The Pot o' Gold is an item that is only available in the mobile version. Once the player has used the pet summoning item, a Pet Leprechaun O'Fyffe will spawn.
One must collect 5 Rainbow Pieces falling from the sky during daytime. Once collected, they can be put together to make the Pot o' Gold anywhere. Due to how common Rainbow Pieces are, it is usually the one of the first things crafted in the game.
In the Android edition, if Terraria isn't updated to the St. Patrick's day update, the Pot o' Gold is named Music Box (Ocean) which means that the item might be on the code of the game.