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
A Boo with a similar color scheme as a Gold Boo appears in Paper Mario: Sticker Star in the safe in The Enigmansion, where it acts identically to a regular Boo, but drops a large amount of coins after the battle.