Bull King is one of the two skins released in the Demon Hunt part of the Journey offer. It could be purchased as part of an item deal for a limited time in September 2016.
Bull King could be purchased for $9.99 in a deal that included 16000 coins. It could also be purchased for $49.99 in the “Best Deal!” which included the Jade Dragon skin and 125000 coins. This skin 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 Demon Hunt sub-offer was released again for a limited time in late April 2019. This time the deal that included 16000 coins was reduced to 3250 coins at $1.99. The Best Deal was lowered to $9.99 with the included coins being reduced to 30000.