Grumpy Frog is one of the three skins released under the monthly Skin'O'Matic offer. It could be individually purchased with game coins in late July 2018.
Grumpy Frog could be purchased in the Skins Shop under the ‘Skin-o-matic’ tab for 490 coins. This skin was available for a limited time and may not be available for purchase in the future.
Details of this skin may vary depending on location, platform or time.
Note: Grumpy Frog was released again for a limited time as part of the All the Internet Things offer in mid November 2019. This time the price for Grumpy Frog was increased to 7000 coins.