How do you get a ho-ho in Pokemon diamond?

You can't get a Ho-Oh in Pokemon Diamond. Ho-Oh is a legendary Pokemon that is exclusive to Pokemon Gold and Silver, and its sequels, HeartGold and SoulSilver.

However, you can find other legendary Pokemon in Diamond, such as:

* Dialga: The legendary Pokemon of time, which is crucial to the story of Diamond.

* Palkia: The legendary Pokemon of space, which is also crucial to the story of Diamond.

* Cresselia: A legendary Pokemon that can be found after completing the main story.

If you want to catch Ho-Oh, you'll need to play Pokemon Gold, Silver, HeartGold, or SoulSilver.