You'll try to find as many Pokémon as you can as you explore Pokémon Brilliant Diamond and Brilliant Pearl. For those looking for each one to fill your entire Pokémon, you have your work cut out for you. If you are looking for Tangela, you will be able to find it in a specific place. Here's where you can find Tangela in Pokémon Brilliant Diamond and Brilliant Pearl.
The only way to make Tangela appear in your Pokémon game Brilliant Diamond and Pearl Brilliant is to complete the Sinnoh PokéDex. You will need to encounter all the Pokémon in the Sinnoh region before you can find it. Once that's done, the National PokéDex will be available to you, so Tangela will start spawning.
You can find Tangela by searching the Grand Underground. Tangela typically spawns in caves with swamps or Bogsung caves. These are the two locations that others have reported seeing this Pokémon. Your best bet is to search these regions and try to spawn a Tangela. If you're having trouble, we recommend that you create a secret base with Grass-type Pokémon statues to increase the chances of this Pokémon appearing in your game. When you capture Tangela, you can evolve into a Tangrowth by teaching it an ancient power and leveling it up once.