Trainers can find many returning Pokémon in Sword and Shield’s Crown Tundra DLC. Some of them are version exclusive, like Bagon representing Pokémon Sword.
Bagon can be found in the Tunnel to the Top, a cave leading to the Crown Shrine. It will only appear in the Sword version, though. Shield owners can only obtain it from Pokémon HOME or by trading with other players.
It might take a lot of time for a Bagon to show up, however. It will spawn around level 65 and is weak to Ice, Dragon, and Fairy-type attacks. Just be careful not to defeat it, though. Try using other types of Pokémon to catch it and make sure to bring plenty of Poké Balls.
Bagon is a Dragon-type Pokémon and has two more evolutions: Shelgon and Salamence. You won’t need any items to make it evolve, it only requires leveling up. It can Mega Evolve using Salamencite, though.