A new feature to this game is the Pokémon Hideaways. There's different biomes in each of the areas of the underground, and you can find wild Pokémon throughout the areas. Many Pokémon are exclusive to these Pokémon hideaways and more Pokémon appear at various points of your progression, with the National Pokédex being the feature that adds the most. Through this, nearly every Pokémon in the National Pokédex to Sinnoh
There are five different pointss where more Pokémon will start showing up:
Get Icicle Badge
Get National Pokédex
With each milestone comes more Pokémon, though the most Pokémon come with the National Pokédex
Each Pokémon also has a chance of having one of their Egg Moves when you catch them in the Underground.
Whenever you enter one of the Hideaways, there is a small chance of a "Rare Spawn" appearing. Rare Spawns are Pokémon not in the Sinnoh Pokédex and can appear at any time.