can you catch a manaphy in diamond
You don't catch it, but you trade the Manaphy Egg over from Pokemon Ranger after you completed the special mission. The Manaphy Egg will start to hatch just like any other egg does, and you have yourself a Manaphy. :3
To get the Manaphy Egg...
1. Complete the Continue Missions in Pokemon Ranger. Afterwards, you should have a new feature on the title screen; Ranger Net.
2. Hold in the R Button, the X Button, and Left on the D-Pad. The option 'Enter the Password' will appear under 'Select a Mission'. Tap the option.
3. On the Password Enter screen, type in P8M2-9D6F-43H7.
4. On the Select a Mission screen, the mission 'Rescue the Precious Egg!' will appear. Accept this mission.
5. After completing the mission, 'Check the Egg' will replace 'Enter the Password' on the Ranger Net menu.
6. Once you're ready, put Diamond or Pearl in the second DS. Turn it on, and go to the Main Menu (Continue, New Game, etc.).
7. In Ranger, tap 'Check the Egg', and tap the screen. Afterwards, tap 'Send the Message', and place the 2 DS' across from eachother.
8. In Diamond or Pearl, a new screen should come up. From here, follow the instructions provided.
9. Congrats, you now have the Manaphy Egg in either Diamond or Pearl! Now, go to any PokeMart (not the department store or pokemon league) in the Sinnoh region and talk to the man in green. He'll give you the egg (Provided you have an open spot in your Party). Now, walk exactly 2,540 steps. After that, it should hatch. You now have Manaphy!