Pokémon Brilliant Diamond and Shining Pearl feature 493 Pokémon and some of them involve special mannerisms. While most simply level up, some require the use of held items. One held item you will need if you plan to complete your Pokédex is the Razor Fang.
To find the Razor Fang in Pokémon Brilliant Diamond and Shining Pearl, you will need to go to the Battle Park. This Held Item can be found in the southwest corner of this area. You can also exchange five Battle Points for the Razor Fang in the Battle Tower. This can only be done in the post game.
Related: Where to find Croagunk in Pokémon Brilliant Diamond and Shining Pearl
The Razor Fang has two uses. This can cause an opposing Pokémon to flinch when the item's holder attacks it. Its main use is to evolve Gligar. It's the only way to evolve the Pokemon into Gliscor, so if you want to complete your Pokédex, you'll need to get one.
Since the item is only available in the post-game, you won't be able to use Gliscor until very late in the game. Still, it's worth getting this item and evolving Gligar just to get the data on it!
For more information on Pokémon, see Where to find Miltank in Pokémon Shining Diamond and Shining Pearl and How to Get All Forms of Eevee in Pokémon Shining Diamond and Shining Pearl on Pro Game Guides.