Mantine is first introduced in Generation II. It is also a dual-type, being Water and Flying-type at the same time. Here is how you can get Mantine and evolve it from a Mantyke in Pokemon Brilliant Diamond and Shining Pearl.
How to Get Mantyke
To get Mantyke, you need to go to Route 223 and Surf on the lake. Keep going until you encounter a Mantyke. Mantyke can evolve into a Mantine under certain circumstances. It has a Swift Swim or Water Absorb ability as well.
How to Evolve Mantine
Before you can evolve your Mantyke, you need to get the Good Rod. To evolve your Mantyke into a Mantine, you need to go to Route 213 first. Use your Good Rod and Fish until you get a Remoraid.
Remorid has a high spawn rate so you do not have to wait for a long time. After getting Remoraid, you need to put both Mantyke and Remoraid in your party. Once they are both in your party, level up Mantyke.
After leveling up Mantyke, it will evolve into a Mantine as long as you have a Remoraid in your party.