Lapras is one of the unique Pokemon that has dual types. This pokemon is both a Water and Ice-type pokemon, and catching one can be challenging. Below is how you can catch Lapras in Pokemon Brilliant Diamond and Shining Pearl.
How to Catch Lapras
Before you can catch the Lapras, you need to beat the Pokemon League first. You will also need the National Dex given by the Professor in Sandgem Town. Go to the Pokemon League and Surf on the lake until you go down the waterfall.
Go to the cave entrance on your right and keep going until you reach the end of the hallway. Use Rock Climb and go to your left and up the cliff until you reach another entrance to your right. Go down the staircase until you reach the next area which is covered in Fog.
You can use your Defog here if you have one. However, you can also just approach the lake to your right and use Surf. This is the area where you can catch Lapras. Lapras has a 10% spawn rate so you may need to explore the lake a few times.