For a limited time, you have the opportunity to capture Charmander, Squirtle, Bulbasaur, and their second evolutions by participating in max raid battles in Pokémon Sword and Shield. These battles vary in difficulty, but they give you the option to add the original starting choices to your Pokémon Sword and Shield collection. Not only can you fight them, but you can fight Mewtwo, unfortunately you can't capture him.
You can locate these three Pokémon the same way you would approach traditional max raid battles. You have to venture into the wilderness area and start exploring the glowing dens all over the region. Those filled with Pokémon should have a distinct glowing light rising in the sky. Approach them and you should see a dark outline of the Pokemon you are about to fight. As you make a black outline, you also see the type of Pokémon before you tap it. Charmander is purely Fire-type, Squirtle is purely Water-type, and Bulbasaur is purely Grass-type, so this should make locating them easier.
Depending on which of the three you are fighting, you have to change your tactics each time. You can find all three of these Pokémon in One Star, Two Star, and Three Star Raids. When you locate a Four Star Raid, you will find their second evolutions, Charmeleon, Wartortle, and Ivysaur, respectively. They don't have five star raids. The new five-star raids have been reserved for Mewtwo, which you can only receive rewards for beating.
Raids for these old favorites will continue until March 2. You have until then to try to capture them. If you miss them, you can always connect some of your Pokémon games to transfer them from Pokémom Home.
