The Pelagers, also known as Homo Sapien Oceanus, is a recognized strain of Abhuman within the Imperium. They are essentially your Merfolk and maybe 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.
But hey, for all their faults and fluff vagueness, they are at least better represented than the Troths.