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 Four Swords, Phantom Hourglass, and Skyward Sword.

Phantom Hourglass
A Rupoor is first encountered in Phantom Hourglass 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.

Four Swords
They also appear in Four Swords with the same function.

Skyward Sword
In Skyward Sword, Link can pick Rupoors up while he's digging during the Thrill Digger minigame and lose 10 Rupees. Using his slingshot on silver crystals in the cave in which the Thrill Digger minigame is played will also yield Rupoors.

Trivia

 * Through hacking, certain pick-ups can be replaced by a Rupoor in Twilight Princess. 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.