Pokemon GO Silhouettes Appear Ahead of Shadow Pokémon Release

Pokemon GO silhouettes are confusing veteran players of the game as they stumble upon silhouettes for Pokémon they've already caught. While this phenomenon has been a bug in the past, this time around Pokémon GO developer Niantic has a good explanation. Here's what you need to know.
What is going on? The first picture, from today, shows a Mareep in silhouette...but I hatched a Mareep back in June 2017. Why is it showing up this way today? Also not the first time (Turtwig and Lapras). @NianticLabs @PokemonGoApp pic.twitter.com/yr2QuJAFgp
— zyxwv098 (@zyxwv098) October 16, 2019
Pokemon GO Silhouettes Appear Ahead of Shadow Pokémon Release
The last time players experienced silhouettes appearing for Pokémon they'd already collected, it was tied to the arrival of Team GO Rocket. The in-game villains had created Shadow versions of several Pokémon, and those version of the Pokémon were causing the Pokémon appear as though they hadn't been caught before.
Now that Shadow Pokémon have returned for Pokémon GO's October celebration, players are once again reporting unusual silhouettes. It appears likely the same phenomenon is taking place.
For the Pokémon not included in the list of Shadow Pokémon, the silhouette effect is likely being cause by their new October-themed costumes.
To clear the silhouettes, simply catch the Pokémon in question. One catch each should do the trick.
Photo courtesy of Niantic/The Pokémon Company