Shrieker is one of the six skins released in the Alien Artshop offer. It could be purchased as part of an item deal for a limited time in early January 2018.
Shrieker could be purchased for $19.99 in the "Best Deal!" which included 25000 coins and 1800 DNA. It was not available for individual purchase and may not be available for purchase in the future.
Details of this skin may vary depending on location, platform or time.
Note: The part of the Alien Artshop offer with the Shrieker skin was released again for a limited time in mid September 2019. The price and the included items for Shrieker remained the same.