What is the white swirl around the Pokémon on the map?
Since the last update (8 December 2017) I sometimes see a Pokémon with a sort of whirlwind around it. In the catch screen, there is a little icon added. When I caught it, it gave me 25 bonus star dust. See the three attached screenshots.
Are there more benefits? What determines what Pokémon get these bonuses?
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Images: Mikhail Nilov, Ethan Wu, Karolina Grabowska, shutter_speed