Rapidash is the evolved form of Ponyta, a Fire-type Pokémon. To get a Rapidash in Diamond, you need to catch a Ponyta and evolve it.
Here's how:
1. Find a Ponyta: Ponyta can be found in the Grand Underground, specifically in areas with grass and rock formations. You can encounter it randomly as you explore.
2. Catch it: Use Poké Balls to catch the Ponyta.
3. Level it up: Once you have caught a Ponyta, level it up to level 40.
4. Evolve: Upon reaching level 40, Ponyta will evolve into Rapidash.
Tip: You can use the "Charm" item to increase your chances of encountering a Ponyta in the Grand Underground. You can get the Charm from the Contest Spectacular in Veilstone City.
Good luck catching your own Rapidash!