Big Mustagu is one of the four skins released in The Fungus Amongus Planet event. It could be purchased in the item deal part of that event in mid January 2021.
Big Mustagu could be purchased for $9.99 in a limited time deal which included packages of 12000 coins, 900 DNA and 1500 mushrooms. It was not available for individual purchase and may not be available for purchase in the future.
Details of this skin may vary depending on location, platform or time.