Thano-nano
For what it’s worth, Thano-nano is not natural. It’s also not exactly unnatural, either — OK, let’s start from the beginning. Once upon a time, there was a sector of space that had two competing space empires, each spanning about a hundred and fifty worlds or so. The two empires fought. The war lasted a while.
Eventually, one of the empires developed a nanotechnology that would allow them to take over freshly-killed corpses and revive them for use on the battlefield. And this worked! It worked so well that the other empire developed the same nanotechnology, and then they both used them on each other until an inevitable ‘Grey Goo’ event wiped out sapient life in both empires. Comprehensively. Neighboring star nations avoided that region of space, and they continued to do so long after the heirs to the heirs to the heirs to the heirs of those star nations had forgotten why. For several million years, in other words.
At the end of that time, humans came by, and of course humans never pass up a chance to go into the Forbidden Places. We were warned, of course. We went anyway. Where we found what had been waiting oh so patiently for us, for millions of years.
And, of course, we’ve been smug about it ever since.
You see, nanotech that’s had the chance to flat-out ‘evolve’ for several million years can get… interesting. In this case, the nanotech developed a symbiotic relationship with the local ecosystems; a colony of Thano-nano would infect dead animal tissue, resurrect it, then pilot it around until it eventually fell apart (usually Thano-nano can get about twenty years out of corpses before they finally collapse into dust). The Thano-nano got biological tissue that would allow it to manipulate the physical world; the various animal species favored for colonization would get protected and cultivated. It was not a bad trade.
The only problem, from Thano-nano’s point of view? Most of the species left after the Grey Goo incident weren’t really great at being tool users. The Thano-nano could function (up to and including limited, sporadic contact with other Thano-nano worlds, on the almost-completely decayed old interstellar communications networks), and Thano-nano got rather good at working around their hosts’ limitations, but it wasn’t ideal. So when human beings started showing up on Thano-nano infected planets, and eventually died on them, the nanotech was happy to actually take over ‘proper’ bodies again.
Fortunately, Thano-nano isn’t actually nasty or belligerent, so only a few Unfortunate Misunderstandings took place before humanity and Thano-nano came to a certain understanding. There are now significant human colonies on at least three or four dozen Thano-nano worlds, and it is understood that those colonies are to provide Thano-nano communities with whatever dead bodies might naturally accumulate. Or blank clone bodies. Those are fine, too. Harder to get in a frontier situation, of course.
It all works out surprisingly well. Thano-nano worlds with human colonies send along shipments of corpses to Thano-nano worlds without human colonies (every Thano-nano world wants a human colony now), and as more and more revenants come online the Thano-nano worlds can do more and more trading for, well, more corpses. There’s an understandable reluctance to let Thano-nano off of their core worlds — what if they mutate again? — but that prejudice may fade, in time. Right now the situation is still quite lucrative for both humanity, and Thano-nano.
It infuriates the other races sometimes, just how stupidly lucky humanity can be. Particularly when the luck is this creepy. Because most of the known Galaxy finds this relationship between humans and Thano-nano to be as creepy as Hell. Everyone else just keeps waiting for the other shoe-equivalent to drop.