The Lucky Horseshoe is the Treasure reward from the safe in 'Neyla is the Key' of Sly Cooper: Master of the Thieves. It is based on the Lucky Charms from the first Sly Cooper game, and it functions like The Guru's Staff Piece from Sly Cooper: Thieves in Time.
In-game description[]
'These strange horseshoe item-like things have been around for years. They were quite common with the Cooper Gang, though, for they always gave them good luck, especially when they had to pull off some tough and/or dangerous heists. And now that you've found one of them, you've certainly become luckier - all damage to the player is now reduced by 50%!'