Golden Cuby is an emote available in-game that you cannot collect from any Egg. This specific emote is part of some Hunt contest started in Reddit, and will be rewarded to 7 winners only.
7 different quests need to be achieved in order to collect it, including specific tasks to complete in-game. Join the discussion over Reddit/Discord to find and hunt them all.
Sharing a screenshot on our Reddit post of you playing against Trainer Golden Cuby in-game can reward you one of the 7 Golden Cuby emotes.