You will need to meet specific evolution requirements for Pokémon if you want them to achieve their final transformation. Many Pokémon in Pokémon Brilliant Diamond and Brilliant Pearl will need an evolution item or know a particular attack if you want to evolve it. For those looking to bring Growlithe into Arcanine, you have to follow a specific method. Here's what you need to know about Growlithe's evolution in Arcanine into Pokémon Brilliant Diamond and Brilliant Pearl.
You will need to catch a Growlithe first. If you have trouble with this, you will only be able to find this Pokémon when you complete the game and have received your national PokéDex. At that point, Growlithe should start spawning in the Grand Underground - the lava caves, sand caves, and Typhlo Cavern. It is also important to note that Growlithe is a Pokémon exclusive to Pokémon Brilliant Diamond.
After getting Growlithe, it's about giving him a Firestone. The best solution to find Fire Stones is to look for them in the Grand Underground through the fossil digging minigame. You can increase your chances of finding evolutionary items by finding the Diglett and Dugtrios that spawn in the Grand Underground passages. When you give Growlithe the Firestone, it should evolve immediately.
If you are playing the Pokémon Shining Pearl version, contact another player who plays Pokémon Shining Diamond. As long as they trade you the Growlithe, you will be able to evolve it in your game using the same method.