How do you get haunter on Pokemon platinum?

You can't directly catch Haunter in Pokemon Platinum. Haunter evolves from Ghastly which can be found in these locations:

* Eterna Forest: You can encounter Ghastly here after obtaining the Cut HM.

* Wayward Cave: Ghastly also appears in this cave, but it's a rarer encounter.

To get a Haunter, you need to follow these steps:

1. Catch a Ghastly: Find a Ghastly in one of the locations mentioned above.

2. Level it up to level 25: Ghastly evolves into Haunter at level 25.

Important Note: Haunter evolves into Gengar at level 45. If you want to catch a Gengar in Pokemon Platinum, you'll need to wait until it evolves from Haunter.