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.
Tritons are one of the playable races introduced to 5th edition in Volo's Guide to Monsters.
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.
|Dungeons & Dragons 1st Edition Races|
|Basic Set:||Dwarf - Elf - Hobbit - Human|
|Creature Catalog 1:|| Brownie - Centaur - Dryad - Faun - Hsiao |
Leprechaun - Pixie - Pooka - Redcap - Sidhe
Sprite - Treant - Wood Imp - Wooddrake
|Creature Catalog 2:|| Faenare - Gnome - Gremlin - Harpy |
Nagpa - Pegataur - Sphinx - Tabi
|Creature Catalog 3:||Kna - Kopru - Merrow - Nixie - Triton|
|Dragon Magazine:|| Cayma - Gatorman - Lupin - N'djatwa |
Phanaton - Rakasta - Shazak - Wallara
|Hollow World:|| Beastman - Brute-Man - Hutaakan |
Krugel Orc - Kubbit - Malpheggi Lizard Man
|Known World:|| Bugbear - Goblin - Gnoll |
Hobgoblin - Kobold - Ogre - Troll