For Pokemon GO’s, Volbeat can currently be found in the Americas and Africa while Illumise can be found in Europe, Asia and the Oceania region.
Prior to the Buddy Up event last week, the opposite was true for the locations of Volbeat and Illumise. Volbeat was originally only found in Europe, Asia and Oceania while Illumise was found in the Americas and Africa. But when the Buddy Up event when live, both Pokemon could be found in the wild in all regions with players even getting a small chance of encountering their shiny forms. After the event ended, Niantic announced that the regions each Pokemon originally resided in switched.
https://twitter.com/PokemonGoApp/status/1255587684673863681
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Where to Find Volbeat & Illumise in Pokemon GO