Region Exclusive: Mr. Mime
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 1st Generation of Pokemon GO, there are four different Pokemon that are Region Exclusive.
One of those four Pokemon is mr. mime, who is exclusive to players only found in Europe. Since Trading has yet to be incorporated in Pokemon GO, the only realistic way of obtaining Mr. Mime is by visiting the greater portion of Europe. 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.