Gold Nuggets are an unused collectible that have data leftover in Fur Fighters, Fur Fighters PC and, most notably, Fur Fighters: Viggo's Revenge.
They work in the same way as Gold Tokens, even turning into Silver Nuggets once collected. However, as opposed to adding a single Token to your count they reward the player with a staggering 25 Gold Tokens.
Although the data exists for them, they can only be seen (as of now) in the equally unused New Quack Subway level in Viggo's Revenge. Here, they also regenerate after a certain amount of time (but only change to silver once the player has been Fluffed).