Rupoor

"You picked up a Rupoor! That means you've lost 10 Rupees. And that's a little bit sad."

- Skyward Sword item description

A Rupoor is a black Rupee-like item that appears in Phantom Hourglass. It is first encountered on the Ghost Ship, after one of the Cubus Sisters advises Link to gather treasure from one of two chests. Opening the chest that the Cubus Sister recommends results in Link finding a black Rupee, a Rupoor. Their only other appearances in the game are on Harrow Island, in which Link can find 10 and 50 Rupoor, and in a maze found in the Isle of the Dead where instead of walls the maze is made of 10 Rupoor and 50 Rupoor pieces. The result of collecting a Rupoor is that its amount will subtracted from Link's wallet. Whenever Link finds one, the traditional item music plays but fades away midway. Link also has a hurt expression on his face when they are found.

In Skyward Sword, during the Thrill Digger minigame, Link can pick this up when he's digging and lose 10 rupees.

They also appear in Four Swords with the same function.

Trivia

 * Through hacking, certain pick-ups can be replaced by a Rupoor. This makes Link lose rupees like in other games. In the actual game, there are no Rupoors, but they seemed to be planned to be included but removed at the last minute. They look similar to the Rupoors in Skyward Sword.