Koalinth
Dungeons & Dragons has often struggled with coming up with unique humanoid races to use in their aquatic regions. After merfolk and tritons, their general tendency is to take an existing race and just put it underwater.
Koalinths are a perfect example of this process, as they literally not defined as anything more than "amphibious hobgoblins". That's it, that's the end-sum of their entire existence; common hobgobbos who just happen to breathe water and swim like fish, who want to conquer the seas the way their kinsfolk want to conquer the land.