Walrus Ball is one of the three skins released under the ‘Happy’ part of the Pet Balls offer. It was available for individual purchase in early June 2017.
Walrus Ball could be purchased in the Skins Shop under the ‘Happy Pets’ tab for 1990 coins. It may not be available for purchase in the future.
Details of this offer may vary depending on location and platform.
Note: Walrus Ball was released again for 490 coins in December 2017 as part of the Best of Agar.io offer.