The designers of Dungeons & Dragons have often struggled with inventing unique humanoid races to use in aquatic regions. After merfolk and tritons, the general tendency is to take an existing race and just put it underwater.
Koalinth are a perfect example of this process, as they are 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, and want to conquer the seas the way their kinsfolk want to conquer the land.
The first print appearance of these monsters was a small paragraph at the end of the hobgoblin entry in the first AD&D Monster Manual. They also showed up in Dragon Magazine #68, complete with a picture of a Lung Wang (which is a hilariously-named sea dragon) and the following sentence:
"[If] 100 or more koalinth are encountered, there will also be a sub-chief present..."
Truly, it paints a fantastic picture of just how unbearable AD&D 1st Edition games must have been.
Ya'aar, Ghosts O' The Salty Marsh has brought the overlooked koalinth into 5th Edition, with at least a little bit of interesting variation in the koalinth sergeant.