There are several shadow Pokémon for you to capture in Pokémon Go. You can only acquire these by battling against a Team Rocket Grunt, one of the three leaders, or Giovanni. A shadow Pokémon has different stats than the original version, but if you’re looking to complete the Sinnoh Collection Challenge, you’ll need to capture a shadow Snover.
Not every Team Rocket member will give you the chance to capture a shadow Snover. Instead, you can only do it by encountering a specific grunt. To encounter them, you can find them at a PokéStop that has been captured by Team Rocket, or they might float above your head in a Team Rocket hot air balloon. These balloons appear every couple of hours, so they’re not the most reliable way to capture a Snover. You’re better off visiting a PokéStop that Team Rocket has.
Each of the Team Rocket grunts that appear in front of you has a specific line of dialogue. The dialogue is meant to identify what type of Pokémon they have and what shadow Pokémon you can receive as a reward. For Snover, you want to battle against the Ice-type grunt. They will have the line of, “You’re gonna be frozen in your tracks.” If you find a grunt that uses that specific line and you win, you can expect to capture a Snover and check it off your Sinnoh Collection Challenge list.