Like hagfish, synx are parasites who will also eat any animal weaker than they. Unlike hagfish however, synx do not rank as 'opportunistic parasitoids' because they actively hunt. There are many creatures that exist as both active predators and active parasites in their nebula, and given the lack of vegetation, this is as close to being "omnivorous" as their specialized food chain gets.
As a Predator
A synx is not the most physically capable animal, so they instinctively and intelligently go after the weakest of the herd: the young. If unable to ghoulishly envelop one of these perfectly packaged meals, a hungry synx will set its sights on an older and larger target, but still likely limit its dining choices to the ill, injured or otherwise incapacitated. If too large, cumbersome or violent to comfortably cram inside their bodies, the synx will pull these meals apart like a child dismantling a doll; eating the limbs first so the torso remains alive for a second meal. To call this death traumatic is a gross understatement, but the younger meals certainly don't have it easier. Seeing one of these large eels swallow down a screaming and flailing child is even more abominable and horrific a scene.
If a grim, endless smile and perpetually vacant expression wasn't enough of a hint, synx share more anatomical similarities to deep sea fish than any other terrestrial animal, a trait obviously most telling by their stomachs. A synx's abdomen appears very unremarkable for its size, and even melds quite well with its deceptively humanoid shape, but like the black swallower the gruesome anatomy here becomes alarmingly apparent during feedings.The entire area of a synx's gut is revealed to be the organ itself; an enormous and thick sack, separated from the surface only by connective tissue and skin. This doughy prison is host to many worm-like parasites, protective alien bacteria and a horrifyingly putrid gastric slurry perfectly befitting a space monster fashioned after an abyssal fish.
This may not surprise you, but a creature with such ridiculous anatomy and gluttonous reputation usually has a great deal of trouble fleeing after engorging itself. This is the most common time synx are caught and killed, as that inelegant retreat offers many opportunities to snare them. On worlds where synx infestations are high, it's an unfortunately common scene to witness crowds surrounding a recently splayed synx as its slayers retrieve a body from within. Onlookers usually gaze in stunned, visceral horror at the display, more so if the retrieved is still alive.
Adults, children and large animals who are removed from synx stomachs are usually quite misshapen and twisted. Like someone having survived a burning building collapse on them, victims usually look twisted, mangled and even "melted" to the point of deformity. The Infants these animals are so notorious for stealing away succumb quickly to the digestive process, leaving little to save. Larger, adult sized meals suffocate quickly in most cases; smothered by the tight fit their own size exacerbates. Individuals between these two age/size extremes have a better chance of survival, and even then it is rare and dependent on a time frame for rescue. Rescue being a term not often used for this since a survivor can expect varying degrees of soft tissue damage, motor function impairment, chemical induced mutilation, possible blindness and even permanent/crippling psychological trauma for the rest of their lives.
As a parasite
Being mostly resistant to digestive secretions, a great deal of pressure and seemingly not needing(?) to breath, a synx can survive for quite a while in a large predator's body, provided such an animal has a strong preference for live (or otherwise fresh) meat. The synx will spread its own alien gut bacteria in the host's body, allowing this bacteria to breed and multiply until the "environment" more reflects the immune response of the synx's body. Obviously this is detrimental to the predator the synx is housed in, and unsurprisingly the process kills the synx's shelter over time. While the host animal remains alive, the synx will steal the largest portion of the predator's intake, and (like the ghoulish things they are) attack and eat any live food that happens to be sent inside as well.
Once it becomes clear the host animal is dying and the food intake stops, the synx will begin to consume its housing partially, before quickly vacating. Synx jaws do not have the capacity to chew and are relatively inadequate at tearing. One stuck in a dying animal, a synx will burrow into the muscle tissue while folding its own gullet inside out. Using its own digestive secretions, this bodily invader will slowly liquefy all the soft flesh it happens to be making contact with as it makes its way out. Once free, synx will usually use some of that same animal's stomach/organ lining as protective cover for traversing the outside world.
The bacteria inside the gut of every synx is the most important component in making alien flesh safe for them to eat. It is not the synx themselves that are super adaptive, but the microbes that evolved over millions of years in their guts as synx hopped from one colossal life-form to the next, always encountering strange, new aliens as their only food source. It is important to note however these miracle microbes aren't infallible, and in the nebula of synx origin the decomposition of animal life is very different from life outside the nebula. The natural decomposition process of most world's biota is simply too alien for this bacteria (and synx biology) to handle, and can be lethal. Any organic matter a synx consumes must be submerged in this stomach bacteria before it has begun to rot, otherwise the flesh will not properly digest and the synx will experience toxic reactions of various degrees. It should be noted that synx have a near equal violent reaction to rotting flesh on the outside of their body as well.
As a combatant and a cannibal
When synx find themselves in exhausting, close range battles, they will also expel their insides, but do so offensively; smothering the face of their adversary with their stomachs. It should be noted the digestive secretions of a synx can also easily eat through their own biological defenses, including their defense to digestion. Synx are naturally cannibalistic and will attempt to eat one another, should any of their kin show physical or mental weakness.