Pokemon Brilliant Diamond & Shining Pearl: Pokemon Exclusive to the Poke Radar

gamerant.com:

The Pokemon Diamond/Pearl remakes, Pokemon Brilliant Diamond and Pokemon Shining Pearl, are very loyal to their source game. Features such as the Underground and the Poke Radar have returned with some improvements.

In Brilliant Diamond and Shining Pearl, the Grand Underground takes inspiration from Sword/Shield's Wild Area, in which players will be able to visibly see Pokemon wandering around. While it's not possible to physically see Pokemon in the grass using the Poke Radar, it's easier for trainers to shiny hunt. Some Pokemon are exclusive to the Poke Radar and can't be found anywhere else in Sinnoh.

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The Poke Radar won't be unlocked until after Cynthia and the Elite Four have been defeated. All the Pokemon in the Sinnoh Dex need to be seen before Professor Rowan hands over the Poke Radar and the National Dex. Once this has been unlocked, Pokemon that don't normally live in Sinnoh will start appearing in the wild.

Thanks to the expansion on Diamond/Pearl's Underground, Poke Radar Pokemon can be found in the Grand Underground, making it easier to catch Pokemon that are extremely rare in the wild. However, the Poke Radar allows trainers to "chain" Pokemon, increasing the chances of getting a Shiny Pokemon or a Pokemon with max EVs.

The Poke Radar can't be used in tall grass (ex: the grass directly outside Solaceon Town on Route 210) or the water. Use Repels to ward off other Wild Pokemon, select the Poke Radar in regular grass, then walk towards any patches that are shaking. Poke Radar exclusives are indicated by patches of grass that shake vigorously and have larger blades of grass.

There's a 2% - 22% chance of Poke Radar Pokemon appearing,