Pokémon Diamond and Pearl/Un-Obtainable Pokémon

You cannot catch any of the Pokémon that are not in the Sinnoh Pokédex until you acquire the National Pokédex. You must see all 150 Sinnoh Pokémon. Once you do so, you must visit Prof. Rowan in Sandgem Town. When you talk to him, Prof. Oak will rush in and upgrade your Pokédex to the National Dex. Once he does so, you can import any Pokémon from FireRed, LeafGreen, Ruby, Sapphire, and Emerald, by putting that game in the DS's GBA slot and selecting the Transfer option at the Diamond/Pearl Main Menu. Once you transfer Pokémon from the GBA games, they will be gone forever from the GBA games. After you transfer Pokémon to Diamond/Pearl, you must then capture them in the Pal Park (directly south of Sandgem Town; go to Sandgem Beach and Surf to the area near the Pal Park).

To see, (in Diamond) or , (in Pearl), you must speak with the Elder of Celestic Town (the old lady in the big house.) She will then show you a picture of Dialga/Palkia, adding the 150th Pokémon to the Sinnoh Pokédex.

Pokémon that cannot be obtained in Sinnoh
Some Pokémon cannot be obtained in Sinnoh. These Pokémon must be transferred from either Pokémon FireRed, Pokémon LeafGreen, Pokémon Ruby, Pokémon Sapphire, or Pokémon Emerald via Pal Park. These Pokémon are listed below.

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Pokémon FireRed / LeafGreen
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Pokémon Ruby / Sapphire / Emerald
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Underground Pokémon
Some Pokémon can only be obtained by going into the underground and doing various things.

Fossils
Fossils can be found while digging into the Underground and can be brought to Oreburgh City's Mining Museum for revival. The Armor Fossil can be found in the Pearl version and the Skull Fossil  can be found in the Diamond version. The rest of the fossils can be found after you obtain the National Pokedex and can be found in both versions. These include the Dome Fossil, the Helix Fossil , the Root Fossil , the Claw Fossil , and Old Amber. Multiple versions of each fossil can be found and revived.

Spiritomb
There are special conditions in the underground that must be met to get a Spiritomb. First, you must obtain the Odd Keystone either by digging around in the Underground or by using the Dowsing Machine in Twinleaf Town. Then, you must talk to 32 players in the Underground.

Note: it doesn't actually require 32 different players; just by talking to players, even if it's just one player, 32 times is all that's required, as long as you leave the Underground after every chat. The precise method for doing this has been tested with two pokemon diamond roms using two R4s, so it should work as well on the original games.

Evolution Stones
Many Kanto and Johto Pokémon are evolved with stones. In the main quest of Diamond and Pearl you only find a few of these stones, more can be found in the Underground. As a result some Pokémon won't evolve until you get the stones from the underground.

Version exclusives
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Diamond version
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Pearl version
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Ruby version

 * — Eterna Forest, Route 203, Route 204 North, Route 204 South, Route 210 East
 * — Route 210 East, Route 229
 * — Iron Island
 * — Route 208, Route 210 West
 * — Lake Acuity, Lake Valor, Lake Verity, Mount Coronet, Sendoff Spring, Turnback Cave

Sapphire version

 * — Route 203, Route 204 North, Route 204 South, Route 205 Eterna City side, Route 212 East
 * — Route 212 East, Route 229
 * — Iron Island
 * — Route 208, Route 210 West
 * — Lake Acuity, Lake Valor, Lake Verity, Mount Coronet, Sendoff Spring, Turnback Cave

Emerald version

 * — Eterna Forest, Route 203, Route 204 North, Route 204 South, Route 210 East, Route 229
 * — Stark Mountain (outside), Route 206, Route 207, Route 214, Route 215, Route 227
 * — Route 224
 * — Acuity Lakefront, Route 211
 * — Lake Acuity, Route 216, Route 217

FireRed version

 * — Route 204 South
 * — Eterna Forest
 * — Route 212 East
 * — Great Marsh
 * — Route 201, Route 202
 * — Route 205 Floaroma Town Side, Valley Windworks

LeafGreen version

 * — Route 204 South
 * — Eterna Forest
 * — Route 228, Wayward Cave
 * — Route 209, Route 214
 * — Stark Mountain (inside and outside), Route 227

Any Pokémon GBA cartridge

 * — Old Chateau
 * — Old Chateau (only in the room with eyes painted on the walls)

The only way to obtain Manaphy is to own a copy of Pokémon Ranger and have completed the game and completed the Manaphy egg mission. Once you have the Manaphy egg on Pokémon Ranger, you can now transfer it over to Diamond and Pearl. Once transferred over to Diamond and Pearl, visit any Poké Mart to receive the egg from a man wearing green.

Phione can only be obtained in game once is gotten. Although Manaphy has egg groups, it is genderless; to obtain Phione, Manaphy must be bred with a. Phione learns almost all the same moves as Manaphy, but has lower statistics. Phione can also be obtained through on the Wii. After expanding the ranch a couple of times, Hayley will ask you to obtain a. After doing this with your Diamond/Pearl/Platinum game, you can trade her the Leafeon for a Phione. Then you can transfer it onto your Diamond/Pearl/Platinum game. (This requires an Internet connection on your Wii & 1,000 - 1,200 points on the Wii Shop Channel.)

Through a Nintendo Event, the player will be able to receive a Member's Card from the same man Manaphy is obtained from. Then, go inside the locked building in Canalave City. Note that you must heal the sick boy before you can do so. A man will tell you that you have a reservation, and you will be forced to sleep on the bed, and wake up on Newmoon Island. At this point, you cannot get off. Walk into the center of the island to meet Darkrai. Alternatively, Darkrai can be obtained in the Japanese versions of the game by using a glitch involving the move Surf inside the Elite Four.

Through another Nintendo Event, you will get a Prof. Oak's letter. Go to Route 224 to find Professor Oak standing at the white rock at the end. He will tell you to give your thanks to someone. Doing so will make Shaymin come, and upon seeing you, run. Chase after it, going along the road until to meet it. Shaymin can also be caught with the surf glitch or the action replay cheat "walk through walls"

Through another Nintendo Event, you will get an Azure Flute. Go back to the Spear Pillar. Upon walking on the first symbol on the floor, it will automatically start playing, and stairs leading upward to the Hall of Origin will appear. At the top is Arceus.