Mexican Skull is one of the two skins released in the second part of the Mr. Bones' Vitality Stash offer. It could be purchased as part of an item deal in mid October 2018.
Mexican Skull could be purchased for $9.99 in a limited time deal that included 25000 coins, 1200 DNA and the Wicked Vendetta skin. 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 second part of the Mr. Bones' Vitality Stash offer was released again for limited times in mid October 2019 and then in late October 2020. The included skin, items and price remained the same for Mexican Skull.