The official Pokémon website has announced that there will be a Field Research for Pokémon GO during November 2018 where players will encounter Shedinja if they make a Research Breakthrough.

Research the Wonderful Shedinja

Don’t get bugged by the Bug types—grab Shedinja and search for a Shiny Caterpie with November’s Field Research.

It might be getting chillier in many parts of the world, but that won't keep the Bug-type Pokémon away in November. Trainers who make a Research Breakthrough during November will encounter Shedinja, a Pokémon that's never been available in Pokémon GO before. Several of Shedinja's slithery Bug-type brethren will also be featured in November's Field Research tasks, including little Caterpie. If that doesn't excite you, perhaps the possibility of the Worm Pokémon being Shiny will!

Trainers have until 1:00 p.m. PDT on November 1 to catch Suicune before Shedinja replaces it, so it's time to start completing a final wave of Field Research tasks if you're still looking for a Suicune. To make a Research Breakthrough, you'll need to collect seven stamps by completing Field Research tasks, and you can only earn one stamp each day. You won't find Shedinja roaming around anywhere in the wild, so don't miss the chance to add it to your collection.

Source: https://www.pokemon.com/us/pokemon-news/research-the-wonderful-shedinja/