The Pelagers, also known as Homo Sapien Oceanus, is a recognized strain of Abhuman within the Imperium. They are essentially your Merfolk and may be found on various Ocean Worlds as they don't really have a listed Homeworld.
They are on equal terms of Neandors when it comes to fluff, as we don't really know that much about them other than the fact that they are Merpeople. At the very least, unlike the Neandors, we could at least extrapolate their use in the Imperium. Due to the fact that they are adapted for aquatic environments, Pelagers make good divers and oceanic farmers, they could even be press-ganged in the Imperial Guard for underwater operations if needed.
Some in /tg/ may like to view it as GeeDubs' way of trying to shoehorn Saharduin into 40k. Some Heretics may even like to fantasize them to be Monstergirls Waifu. Although the reality is maybe as mundane as Kevin Costner's Waterworld. Or something as hideous as the abominations of Dougal Dixon's Man After Man. Whatever it is, Pelagers are going to be all smooth, slippery and downright fishy.
With the release of Liber Xenologis, it is basically all but confirmed that Pelagers are literal Sea Ogryns, so no hot waifu mermaids for us, unfortunately. Since they all hail from Ocean Worlds, Pelagers developed gills to breathe in 'any liquid' and scales which is, as you can imagine from even basic biology, utter bullshit. But whatevs, GeeDubs gonna GeeDubs. Due to the fact that they live under the intense pressure of the watery depths, they possessed iron-hard muscles to withstand pressures that would crush any standard human, as well as - somehow - surviving standard pressure without exploding from sudden decompression. The bright side is that you can now put Pelagers on tabletop by literally using Ogryns and painting them blue, so go wild.