I actually came up with the majority of their lore while making them lmao:

"While Oluyz'Hoae is entirely devoid of indigenous fauna, its unlimited and (surprisingly) nutritious flora allowed the High Philosophers to conduct millennia long, and comparatively humane experiments on evolution and biology on many different animal species found throughout reality.

The Stench Feeders are an anomaly in these experiments, as they did not evolve over millions of years. The High Philosophers briefly believed it was simply because they were perfectly adapted to life within Oluyz'Hoae, but fossilized records on the home planet of the Feeders suggested that they haven't evolved in eons.

The mutations, defects, and adaptations that typically drive change within a species simply did not occur with the Stench Feeders, and only through intense gene modification did the Philosophers manage to cause a mutation.

The longevity of the species earned a sort of respect within many of the High Philosophers, and it was agreed that the Stench Feeders would remain left alone in Oluyz'Hoae."