Pokemon GO just introduced a new tier for Eggs: Strange Eggs.
These Eggs require the highest walking requirement ever of 12 km in order to hatch a Pokemon.
They can be acquired during an in-game event from October 12 to October 19 (though it’s possible that the Eggs will be available after the event is over) by defeating Team GO Rocket leaders, according to a blog post from Niantic. You’ll need to beat six Team GO Rocket grunts and collect their Mysterious Components and build a Rocket Radar before you can find a Team GO Rocket leader. You need at least one free space in your Egg inventory to get the Strange Egg.
The Pokemon that hatch from Strange Eggs are either poison-type Pokemon, dark-type Pokemon, or Pokemon that evolve into one of those types.
Here are the Pokemon that can hatch from Strange Eggs, according to The Silph Road:
A new Special Research quest has gone live with the in-game event, which will let you battle Giovanni and catch a Shadow Mewtwo. This is the first time since Pokemon GO Fest that players can catch Shadow Mewtwo. According to Niantic, this will be the last Special Research task involving Team GO Rocket.
The following Pokemon will appear more often during the event, according to users on The Silph Road: Spinarak, Houndour, Poochyena, Gulpin, Stunky, Purrloin, Skorupi, Nuzleaf and Croagunk.
You can also find Team GO Rocket in PokeStops and balloons more often. Diglett, Shellder, Slowpoke, Aerodactyl and Skarmory have been permanently been added to the lineup of Shadow Pokemon you can get from Team GO Rocket grunts. In addition, the Team GO Rocket leaders use Shadow Growlithe, Drowzee and Omanyte, and you can find Shiny versions of each of these Pokemon for beating the leaders.