How can you nick name your Pokemon on diamond after caught it?

You can't nickname Pokémon directly after catching them in Pokémon Diamond. Here's why and how you can nickname them:

* No Immediate Nicknaming: Unlike some other Pokémon games, Diamond doesn't allow you to nickname a Pokémon immediately after catching it.

* The PC is Your Nicknaming Hub: You need to go to the Pokémon PC in any Pokémon Center. Once there, you can access the "Summary" of the Pokémon you want to nickname.

* The "Change Nickname" Option: In the Summary screen, you'll see an option to "Change Nickname." Click on this, and you'll be able to give your Pokémon a new name.

Let me know if you have any other questions about Pokémon Diamond!