Region Exclusive: Seviper
As like the Mainstream Pokemon Games where some Pokemon are exclusive to a given Version of a Game, so something similar is found within Pokemon GO. Instead, however, a handful of Pokemon are exclusive to a given region or country in the world. Within the 3rd Generation of Pokemon GO, there are a whopping nine different Pokemon that are Region Exclusive.
One of those nine Pokemon is seviper. When the 3rd generation first came to Pokemon GO, Seviper was exclusive to Europe, Asia, and Oceania (Australia & Surrounding Islands). However, after about a month of this, Seviper suddenly became exclusive to North America, South America, and Africa instead, the same where it still can be found. Experts aren't sure if Seviper will continue to rotate back and forth between these given countries, but Niantic could certainly continue to do this so players across the globe can find Seviper.
Since Trading has yet to be incorporated in Pokemon GO, the only realistic way of obtaining seviper is by visiting one of these given areas. Sometimes there may be given Events that are held which will see certain Region Exclusive Pokemon being found in other areas of across the globe, but said events are unpredictable. It was also once believed that a player could find Region Exclusive Pokemon of other countries in Pokemon Eggs, but that concept has since been deemed impossible.