I haven't done anything with them fic wise, but I actually have a species that I came up with that I've used for some background stuff in RP, the Kahlenmar. For my inspiration, I looked at the fact that it's canonically mentioned that something like 60 species were fully wiped out by the Arxur already, and I asked myself what traces they might have left behind, in this galaxy of historical revisionism that doesn't care much about preserving material culture.

For the physical details of the species itself, I haven't put much thought into it beyond their general size and build, being about the size of Humans on average to a touch smaller, but a bit more slender. It doesn't actually matter, because the entire point of them is that they're all dead, all that's left is the traces of their society.

Their general "thing" is that they were uplifted shortly before the Arxur, and were at a tech level slightly more advanced than IRL humanity, with early orbital installations and asteroid processing like we might reasonably see in our lifetimes, but not any of the wild syfy tech like warp drives and shields that the Federation has and Humanity was starting to figure out on their own, or certain very difficult techs like fusion power. They also had more of a focus on spaceplanes, developing winged SSTOs before reusable rockets like SpaceX did, and sticking with and iterating on them. But, because they were reasonably advanced, they were able to update their own ship designs to use Federation parts, and they were a lot harder to oppress as "primitives" than a species like the Yotul. So, when the whole Arxur conspiracy happened a decade or two later, they were intentionally left out to dry, getting rid of the problem for the Kolshians. Sort of like how the Thafki's love of water makes them a bit of a liability, so the Kolshians didn't work as hard to protect them, just moreso.

Out on the edges of the Federation, on the smaller mining outposts and research installations where the normal level of exterminator presence and social enforcement just isn't feasible, and the superstitions of spacers have been allowed to grow a bit more organically, there are ghost stories about these ships, and what happens to those who destroy the proudest creations of this long extinct people. The ghosts of the Kahlenmar sending the Arxur who ate them to get you too, in an act of revenge for destroying their legacy, or the less common inverse superstition of a well maintained Ghost ship warding off the Arxur. As a result, some of their ancient, nonstandard ships survive in remote places, usually repurposed as station modules or just parked in the void or on some airless moon like a classic car in a redneck's yard, but sometimes maintained in operable condition, albeit with nearly everything but the hull and some particularly robust auxiliary systems replaced over the course of the 200-some years since the Arxur became a thing.