Viper Dragon is one of the three skins released under the Royal Pack part of the House of Dragons offer. It could be purchased as part of an item deal for a limited time in late September 2017.
Viper Dragon could be purchased for $19.99 in the "Best Deal!" which included the Razor Dragon and either 50000 coins (Browser) or 1800 DNA (Mobile). It was not available for individual purchase and may not be available for purchase in the future.
Note: The Royal Pack sub-offer was released again for a limited time in late May 2019. This time the Best Deal was lowered to $9.99 and the included coins were reduced to 30000.