Silver Tusk is one of the three skins released under the Dawn of Man part of the Stone Age offer. It could be individually purchased using game coins in mid May 2017.
Silver Tusk could be purchased in the Skins Shop under the 'Stone Age' tab for 1990 coins. It was available for a limited time and may not be available for purchase in the future.
Details of this offer can vary depending on location, platform or time.
Note: The Dawn of Man sub-offer with the Silver Tusk skin was released again for a limited time in early November 2018. The price for Silver Tusk remained the same at 1990 coins.