Gold Boo

Gold Boos are golden variants of Boos.

New Super Mario Bros. 2
Gold Boos first appear in New Super Mario Bros. 2 when Mario or Luigi touch a Gold Ring. They begin to flee upon turning gold, leaving a trail of coins behind for the player to pick up. Just like all the other golden enemies, they give five coins to the player upon defeat. Big Boos also have the same effect and turn into Big Gold Boos.

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.

Mario & Luigi: Superstar Saga + Bowser's Minions
Gold Boo appears in Minion Quest: The Search for Bowser, when the player reads Boo's amiibo, it appears in its golden form.