PokeMiners on Twitter released information about a new set of Shadow Pokemon that was updated to the Pokemon GO GameMaster. With any leaks or datamining though, players should take into account that any information could change until Niantic makes an official announcement. However, it doesn't hurt to anticipate the new additions.
There are 10 new Shadow Pokemon that will be added in total. Based on the list, there doesn't seem to be much of a pattern for the 10 new additions, but that could always change.
10 new Shadow Pokemon in Pokemon GO
Within the comments and replies to the post on Twitter, many players did notice one trend with the new roster of Shadow Pokemon. Many of them are viable picks in the Pokemon GO Battle League, and there is already some fear of a Shadow Granbull with a Charm fast attack.
How to get the new Shadow Pokemon in Pokemon GO
It's impossible to guarantee how to get the new Shadow Pokemon until Niantic officially announces the information. However, if they are anything like the rest of the Shadow Pokemon in Pokemon GO, then attaining them should be fairly straightforward.
Shadow Pokemon are caught by taking on grunts, Team GO Rocket leaders, and Giovanni himself. It's possible that the leaders will get the new Shadow Pokemon, or even Giovanni, but it's most likely the grunts that will have them in their rotation.
In that case, players will just need to hope they get the right grunt encounter, and then defeat them to have a new Shadow Pokemon encounter.