Not so Merciful
An impressive ability for concealment and blending in makes synx exemplary stealth hunters, but this is not the defining characteristic of their disingenuous nature. Once enough physical contact has been made, any animal or person finding themselves in a violent altercation with these creatures will quickly notice parts of their body have been robbed of the most vital survival component a life form can have: pain. The ability to cause great harm to a victim whilst they remain unaware is a dastardly ability, one that could be analogous to disarming a security system before robbing a vault. This is done through quite a potent and nasty toxin delivered through bites and scratches. This toxin does not necessarily numb the body, but instead causes actual nerve damage and inhibits the ability feel pain. The effect of exposure can take hours to subside, but some degree of damage will be permanent. This is usually not an issue though as most people who come in contact with this toxin are eaten alive anyway. Depending on fear, most prey items a synx consumes remain passive throughout the ordeal. The initial experience lacks the violent spasms agony calls forth, and the adrenaline regular physical altercations would spawn. As pleasant as this is for the synx, it is far from intentional.
Synx cannot help but share this toxin with whatever they have distinguishable contact with, it is part of their biological makeup. The purpose of this anatomy is parasitic, and they possess it to incite similar lax responses in the larger animals they burrow into. Though synx are very malleable animals, they are still organic and therefore limited in how much stress they can take. Having an elephant sized beast thrashing around violently as something crawls through a wound into its body wouldn't be very safe; the muscle spasms alone might crush a vital organ of the intruder. With their toxin, a synx can burrow into any soft flesh without this worry, and often the animal - though uncomfortable - has no actual idea of what transpired. Texture wise, most of the exterior of a synx varies between marshmallow-like too something more resembling a raw, plucked bird, but their digits and tail-tip have a considerably more bristle-like feeling. Those bristles are actually thousands of tiny tendrils, similar to the stingers of a jellyfish, but some degree smaller. When active these tendrils utterly destroy soft flesh with an almost acidic appearing result, and with the use of the more narrow parts of their body, a synx can worm their way through meat quite fluently. This ability has its limitations as well, and when a synx finds itself de-limbing a person, they have to severe the limbs between the bones, as they cannot go through them.
Home sweet home
When inside large carnivores there is usually nothing too dense between a synx and the beast's entrails, it's just a matter of finding them. Once inside, this parasite will make itself a little home, stealing quite the large portion of its hosts intake. It's worth noting that this is a synx's natural environment. While sunlight and allergens kill these critters in alarming frequency, typical digestive acids, airless depths and suffocating muscular contractions don't seem to affect synx in the slightest; they appear to thrive in it. Unfortunately the life of a parasite is a destructive one, and in a matter of months (if not much sooner) the host will die a slow death of malnutrition, or toxic shock from the synx's leavings and bacterial spread. Unable to live in a dead body, the synx will tear its way out, use organ tissue to protect itself, then look for another suitable host or something helpless to eat.
--The thick tuft of fur-looking material they have on their bodies and neck is not hair at all. This spider-web like substance is sticky and pulls off quite easily. When exposed to moisture it clumps and melts very quickly, becoming an adhesive that eventually solidifies.
--What little they have of bones consists of a mere cartilage frame. The purpose of this frame is to act like a spring, allowing a synx to quickly bounce back into its humanoid shape, even after the most extravagantly awkward positioning. This frame also encourages and guides them into taking a humanoid-like form to begin with, as it is unlikely they would at all without it.
--The closest thing to actual hands this creature possesses are its jaws, which possess dexterity and movement comparable to human hands. Their heads can also collapse and separate even more easily than a snake's, thanks to this unique skull.
--Synx do not seem to breathe. It is unclear how they get their oxygen (assuming they need it) however even without taking in breaths a stationary synx can be anything but. The thick and clear goop that they have for blood doesn't easily travel through them, and it takes six small hearts throughout their body to pump it through their innards. A synx resting after a lot of activity can be seen slowly pumping in waves that travel through its body, though a lax synx resting may be entirely frozen.
--These creatures do not heal from injury anymore impressively than any other animal, but they possess one excessively regenerative component. Under the eyes of a synx is another partially developed set of eyes. Like a shark losing its teeth, synx have to biologically keep spares of what they are guaranteed to lose, and given the violent nature of their feedings, they all lose eyes. If a synx doesn't lose its eye, at some point it will have to shed the old one to make way for the new. This is even more unpleasant than it sounds...