From 1d4chan
Maybe a little bit badass.

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.

The two best sources on Koalinth lore to date are the article "Heroes of the Sea" in Dragon Magazine #250 for "generic" koalinths, and "Sea of Fallen Stars" for Forgotten Realms-specific lore.

In Heroes of the Sea, it is stated that koalinths stand about 6'6" tall, with sleek, muscular bodies. They have webbed digits, gill-slits in their necks, and light green skin covered with patches of dark green, aqua or blue hair. Whilst clearly related to hobgoblins, they have a seal-like facial structure, and male koalinths share the distinctive nose (and aggressive behavior, and loud bellowing) of male elephant seals. Like hobgoblins, they live in rigidly organized, militaristic tribal societies, and their tribes are constantly at war with each other. Whilst PC koalinths are likely to have left their culture after realizing how pointless the constant tribal squabbling is (or else they're just mercenaries), their culture shapes them and they tend to think of life in military terms, often using military jargon to describe everyday life. Also, they have a fear of fire and a superstitious dread of giant squids.

In the Forgotten Realms, whilst koalinths of the present are largely just "hobgoblins, but underwater", it's stated in the aforementioned Sea of Fallen Stars splat that they used to be a prouder, more advanced race - described as "akin to what the locathah are today". However, they descended into barbarism and never escaped after the sea elves launched a genocidal campaign against them when they tried to, quote, "rise above their barbaric status and join the civilized races". Only their ability to freely breathe both freshwater and saltwater, allowing them to retreat through the estuaries from the coastline, saved them from being annihilated. As a result of this onslaught, and their descent into an endless cycle of vendetta and "might makes right"-fueled infighting, the koalinths are all but assuredly stuck as primitives whose culture has little skill at anything beyond raiding, hunting, gathering, and crude tool-making. As a result, the koalinths of Faerun absolutely despise sea elves, and love to capture them so they can be tortured to death and then eaten. A popular superstition is that eating a sea elf's heart will grant supernatural good luck, although they also believe this of any single or collective foes that have repeatedly fought them and triumphed.
So, in short, koalinths in the Forgotten Realms could have been a good guy race, but they ended up as generic tribal baddies because the sea elves were dicks. Nice job fucking it up, elves! Oh, and they didn't learn from this, because the exact same lore can be applied to the merrow, or sea ogres!

PC Stats

Ability Score Minimum/Maximum: Strength 6/18, Dexterity 6/18, Constitution 5/18, Intelligence 3/18, Wisdom 3/18, Charisma 3/14
Ability Score Adjustments: -1 Charisma
Racial Class & Level Restrictions: Fighter 11, Cleric 9, Shaman 7, Witch Doctor 7
Available Kits: Tribal Defender, Pit Fighter, Sellsword, Shaman, Witch Doctor, Oracle, War Priest, Wandering Mystic
Natural Armor Class: 10
Special Advantages:
Amphibious: Koalinths can breathe both air and water, and move on both land (MV9) and underwater (12).
Water Adapted: Koalinths suffer no movement or attack penalties when in water, and receive Swimming as a bonus proficiency.
Special Disadvantages:
Water Dependent: A koalinth can survive on land for days equal to its Constitution score, but every 2 days out of water inflicts a -1 penalty to its ability and proficiency scores, with death occurring if Str, Dex or Con drops to 0.
Weapon Proficiencies: Awl Pike, Harpoon, Javelin, Military Fork, Partisan, Ranseur, Spear, Spetum, Trident
Nonweapon Proficiencies: Chanting, Close-Quarter Fighting, Direction Sense, Hiding, Intimidation, Looting, Religion, Wild Fighting