Even as part of the original 151 Pokemon, Dratini is still one of the most sought after Pokemon in Brilliant Diamond and Shining Pearl.
The Dragon-type has been a fan favorite since the series first introduced the Kanto region all those years ago. Dratini evolves into Dragonair and then into Dragonite, making it a must have for any Pokemon fan.
In Pokemon Brilliant Diamond and Shining Pearl, Dratini can be encountered by fishing with a Super Rod on Mt. Coronet or in the Fountainspring Cave biome of the Grand Underground.
How to catch Dratini on Mt. Coronet in Pokemon Brilliaint Diamond and Shining Pearl
Pokemon Brilliant Diamond and Shining Pearl trainers will need to beat the Elite Four, become Champion, and unlock the National Pokedex by registering every creature in the Sinnoh Pokedex.
Once that is done, Dratini will be available to catch. You will need the Super Rod, which is obtained in the Battle Resort near the Route 225 entrance. This also can only happen after becoming Champion and acquiring the National Pokedex.
Go to Mt. Coronet after you have done the above and look for any body of water. Cast your Super Rod and if something bites, it has about a 20% chance to be a Dratini. Battle it like normal and use whatever form of Poke Ball you want.
Where to find Dratini in the Grand Underground
Finding Dratini in the Grand Underground of Pokemon Brilliant Diamond and Shining Pearl will also require the National Pokedex to be obtained after you become the Champion.
There are 50 types of Pokemon that can spawn in the Fountainspring Cave biome, so you could be looking for Dratini for a while. Just exit and reenter as many times as it takes for one to appear.
When it does appear in the Grand Underground, just walk into it to start the encounter. Its level may range anywhere from 16 to 63. Take its HP down, throw a Poke Ball at it, and hope it stays caught.