This is a very similar concept to the Alolan Forms introduced in Sun and Moon. The idea is that some pre-existing Pokémon have completely different appearances because they’ve adapted to Galar’s unique environment. You’ll find these new Galarian Form Pokémon naturally in the wild, while the original forms cannot be obtained (although there are exceptions).
Galarian Form Pokémon share the Pokédex numbers of the original Pokémon, but their stat distribution, types and Ability are usually different. They are technically the same Pokémon, but functionally, they might as well be different Pokémon. A new feature is that certain Galarian Form Pokémon can evolve into brand new Pokémon.