Legendary Dogs (Raikou, Entei, and Suicune) are exclusive to Brilliant Diamond, and Legendary Birds (Articuno, Zapdos, and Moltres) are exclusive to Shining Pearl. You will be able to get these legendaries in the post-game. To get them, you will need to get Registeel, Regirock & Regice first. This guide shows how to get Registeel, Regirock & Regice, and Legendary Dogs/Birds in the Pokemon Brilliant Diamond & Shining Pearl (BDSP).
How to get Registeel, Regirock & Regice
To find Registeel, Regirock & Regice, you will first need to unlock the National Pokedex. After finishing the Pokemon League, and registering all 150 Pokemon to the Sinnoh Pokedex, go and talk to the Professor in Sandgem town to get the National Pokedex. Click here to see how to Unlock the National Pokedex.
Find 1xMysterious Shard L or 3xMysterious Shard S
Now go to the Grand Underground, and look for the locations marked with a yellow diamond on the minimap. Pressing ‘R’ at such locations will reveal a glowing spot on the wall. Dig the wall at this spot until you find 1xMysterious Shard L or 3xMysterious Shard S.
Buy a Discovery Slate
Go to the Ramanas park on Route 221 as shown in the picture below. Enter the building and go to the lady on the left side and purchase a Discovery Slate in exchange for 1xMysterious Shard L or 3xMysterious Shard S.
Get Registeel, Regirock & Regice
Once you have a Discovery Slate, go through the door on the other side of the room and enter the Discovery room in Ramanas Park. Place the slate on the pedestal in the Discovery Slate as shown in the picture below. This will make Registeel, Regirock, or Regice spawn. Keep repeating the process again to get the remaining two Pokemon.
How to get Legendary Dogs/ Birds
Once you have Registeel, Regirock, & Regice, go back to the Grand Underground and search for 1xMysterious Shard L or 3xMysterious Shard S. Then go back to Ramanas Park and purchase Kanto Slate. Now go through the door on the other side of the room and enter the Kanto room in Ramanas Park. Place the slate on the pedestal in the Kanto Room as shown in the picture below. This will make Legendary Dog/ Bird appear depending on the game version you have. Keep repeating the process again to get the remaining two Pokemon.