What trainer has a hippowdon in Pokemon diamond?

There is no specific trainer in Pokemon Diamond who uses a Hippowdon. Hippowdon is a Ground-type Pokémon that evolves from Hippopotas, which is not found in the wild in the Sinnoh region.

To get a Hippowdon in Pokemon Diamond, you would need to:

1. Trade for a Hippopotas: Obtain a Hippopotas from someone who has caught one in another region.

2. Breed a Hippopotas: Breed a Hippopotas with another Pokémon, ensuring that the offspring inherits the Ground-type egg group.

3. Catch a Hippopotas in another game: If you have access to other Pokemon games where Hippopotas is found, you can catch one and transfer it to Diamond.

There are no trainers who use Hippowdon in Pokemon Diamond, but you can obtain one through the methods mentioned above.