A great distance from most sapient life resides a galaxy; one most star faring vessels cannot reasonably reach, not that most species want to. A dense nebula engulfs two quadrants of this galaxy, illuminating the night (and day) sky of all close by worlds like an extravagant, pollution-induced sunset. This nebula has been given many names, oddly none of these compliment its size or beauty, and some of the less flattering ones sound outright ominous. One only need see the eerie things that teem inside to know why this place is so infamous and avoided. It is full of life, and quite a bit of it at that. This is not space-dwelling life, but life that evolved and exists only here; treating the area of the nebula like a galactic fish bowl. Fish is quite the appropriate word to use since this ecosystem greatly resembles the beginnings of life on so many worlds, or perhaps the more familiar dark abyss of modern day terrestrial oceans.
The only difference is the scale, and as you can imagine a galactic sized ocean = galactic sized fish. Some of these fish have equivalent mass of small planets, and even generate their own gravity. The movement of these animals is also unapparent to most eyes, and can easily be compared to the movement of clouds. Much like any lifeform you are familiar with, these animals have much smaller animals existing on top of them; microscopic in comparison. By our perception (and scale), this would be a separate ecosystem of life existing on (and in) these gargantuan beasts. These ecosystems have their own environments, thick bacteria ridden atmospheres and even a familiar food-chain of predators hunting prey. Like with our world, one particular animal in these food-chains stands out.
Unlike earth, it was not a primate giving the other wildlife trouble, but instead an anaconda sized, parasitic worm. Through circumstance of survival and intelligence, this parasite became the dominate animal on most of the nebula dwelling fish, and like a nasty case of lice, these worms spread to each larger animal their host came in contact with. These white, elongated, convulsing masses parasitized the other predators that existed on the nebula fish, living in their entrails for a time. Obviously this was destructive to the predators they inhabited, and the existence of this worm was put in jeopardy due to the high fatality rate their gluttony was starting to spawn. Eventually, thanks to millions of years of violent evacuations, these worms adapted to survive in conditions outside of a body. They still existed as parasites, but they were no longer sentenced to death when the host died, at least not immediately. Instead they used their borrowed time to emulate the act of predation; attacking and attempting to consume anything more meek and helpless then themselves. This turning point was early in their evolution, so their body had lots of opportunity to adapt to this new lifestyle. Millions of more years later and these parasites are now more eel-like then worm, and they are now also genuine predators... as much as an evolved, alien tapeworm can be, anyway.
It is unknown how synx get from their nebula to inhabitable worlds; they possess no technological capabilities. Even so, they can be found on most civilized and even pre-industrial worlds. Many of these planets have but a few synx, but some unfortunate worlds boast a few hundred thousand. Synx populations on alien worlds seem completely random and without pattern.