Triton

Fish man. Merman?

Tritons (not to be confused with tridents) are a race of aquatic humanoids originating from Greek mythology, being followers/descendants of the original Triton, the son and messenger of Poseidon. They can essentially be summarized as "merfolk, but with a sillier name".

In Dungeons & Dragons, Tritons are distinguished from merfolk in that they have two lower limbs rather than one (originally two merfolk-style fishtails in lieu of legs in their 1e Fiend Folio appearance, thereafter changed to the less silly legs with finned feet look) and are more magical; Tritons originated on the Elemental Plane of Water, but have since migrated en masse to the Material Plane, to the point they are considered Native Outsiders in 3rd edition.

Tritons are fairly obscure, but they have been playable races throughout the first 3 editions, getting racial writeups in 1e's "The Sea People", 2e's "Sea of Fallen Stars" for the Forgotten Realms, and 3e's "Savage Species". This is because, unlike merfolk, the tritons' legs allow them to freely move about on land and in the water with equal ease, whereas merfolk are pretty much reduced to crawling like oversized slugs if you pull them out of the water.

5th Edition

Tritons are one of the playable races introduced to 5th edition in Volo's Guide to Monsters. They are very paranoid of other races, having spent the entirety of their lives underwater with little to no contact with the outside world. They act proud and noble, but are like fish out of water on the surface. Almost no one sees them or cares about what they do, so no one takes their proud boasting about their culture seriously. This can lead to tensions as your Mary Sue Triton gets humbled by surface dwellers who have no idea who he is.

Statline

Alignment – They tend towards lawful good and think of themselves as the guardians of the prime material plane. Tritons have a culture of benevolence and order.

Ability Score Increases – Strength, Constitution and Charisma scores increases by 1 of your Triton character.

Amphibious – Your triton character can breathe water and air.

Resistant to Cold Damage

Speak to Sea Creatures

Control Air And Water – With this trait, you can cast Fog Cloud. And at the starting of 3rd level, you can cast Gust of Wind with this trait. Moreover, starting at 5th level, you can cast Wall of Water with it. After using this trait to cast a spell, you can’t cast with it again without finishing long rest. For these spells, Charisma is your spellcasting ability.

Magic the Gathering

They are also used in Magic:the Gatherings Theros setting. Here they are much more directly merfolk with legs with a funny name, as they are literally merfolk in creature type. Cosmology isn't particularly related to D&D's as they are not more extraplanar than any other humanoid.