Primal 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.
Primal could be purchased in the Skins Shop under the 'Stone Age' tab for 2900 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 Primal skin was released again for a limited time in early November 2018. The price for Primal remained the same at 2900 coins.