Aetillians

The Aetillians have a typical land-dwelling upper body, with gaseous oxygen processing lungs. Their gills are merged internally with their lungs, and thus breathe under water via inhaling water through the lungs, which is then 'exhaled' out gill vents on their back. This has given them the nickname of 'sirens' in myth, as these gill vents can be used to produce deep bellowing songs, by passing air through their mouth and out their gill vents. They can also utilize these vents to pressurize the water internally and expel it with enough force to propel them through the water. The Aetillians spend their lives adrift in familial packs which make up large ocean communities. They are mostly nomadic and very few permanent Aetillian villages exist in the open ocean. Most of these settlements are based around small bays and atolls which provide natural protection from the open ocean and lend to the ability to settle down. Because of this, Aetillians are not an advanced race of Merfolk. Most, if not all of their clothing, armor and weapons are made from natural sources or cobbled together via scrap and salvage they've collected from the ocean floor. This is also reflected in the few Aetillian settlements, which could be seen more as ship graveyards than villages to the casual observer.
As mentioned before, Aetillians are the source of the Siren myth among sailors. When sailors would come upon small packs of Aetillians sunning on beaches or rocks, the Aetillians would utilize their ability to bellow air through their gill vents, which would produce varying sounds from whistles to deep bellowing noises, similar to whale song. It is thought that via this ability they are also able to communicate with sea life, especially Whales and porpoises. This theory is bolstered by the fact that Aetillian Merfolk have been seen utilizing sea life as means of transportation/manual labor.
Aetillian language is an interesting mix of guttural noises and consonant only phenomes. It is really only able to be produced properly by the Aetillians, with their unique biology. Despite this, there are those who have learned Aetillian and speak with some proficiency, well enough at least to have set up trade and alliances with varying Familial packs.