Gold Boo

Gold Boos are golden variants of boos that appear when Mario touches a Gold Ring.

New Super Mario Bros. 2
Gold Boos first appear in New Super Mario Bros. 2 when Mario or Luigi touch a Gold Ring. When they become golden they begin to run away leaving a trail of Coins behind for the player to pick up, just like all the other golden enemies upon being defeated they give five coins to the player, Big Boos also have the same effect.

Paper Mario: Sticker Star
The Gold Boo reappears in Paper Mario: Sticker Star only one appears in the safe in The Enigmansion, here it's battled like a regular Boo and can be defeated with any attack, once it's defeated it releases coins for the player to pick.