Tough is one of the five skins released under the Skins part of the Agar.io Birthday offer. It was available for individual purchase in late July 2017.
Tough could be purchased in the Skins Shop under the ‘Birthday Special’ tab for 3320 coins. However, this skin may not be available for purchase in the future.
Details of this offer may vary depending on location and platform.
Gallery
Tough - Original Image