A changeling can refer to two different things in the context of CTV mythology.
(posted from Lore-tier patreon by author) Perhaps you have heard of Changelings in fairytales, those Fae impostors who switch out an infant mortal for a glamorized replacement of either enchanted wood or a wizened fairy which fakes his demise shortly after the trade is complete. The fate of the young mortal is up to the whims of the Fair Folk who took it, either to be raised in the courts as a servant or pet, or to be whisked away to pay tithe to one fiendish entity or another. Those infants snatched away in this fashion can be recovered by trickery, a puzzlement that causes the ersatz infant to drop his disguise in incredulity. Or, more horribly, the infant is tossed in the fire with the logs under suspicion of being a changeling. Pretending to brew in an eggshell is a bit less of an extreme option for finding out what is what. The glamour faded, as if by some strange honor system, the baby should be returned shortly after the creature departs in a rush (usually up the chimney).
These changelings, are not necessarily shape-shifters, but masters of disguise, glamour and beguilement. These are the Changelings that are Fae in origin, and Changeling is less of a species/race name and more of a description, the way one describes one who swims as a swimmer. Any Fairy or Goblin could be a Changeling if they were so inclined or employed, should they have a reason to. By and large, Fairies and such do not have much opportunity to swap out human babies these days as far as Kellwood is concerned, it's a long travel to the Human City and not worth the effort.
Now that we have established what a Changeling is, let us discuss the variety living in Kellwood. There is a specific Demon called a Changeling, and this is what conjures to mind when the word is uttered. Often they are called Changeling Demons to be specific, but mostly in Fairy lands where Changeling still carries the original connotation.
While they operate similarly in targeting the young, the similarities somewhat end there. Changelings are a brood parasite. And unlike the Changelings of fairy kind, there's no getting the baby back after mama Changeling has visited the crib.
Changelings are born in quintuplets, either all male or female. The mother will bring her brood along to find a host parent, typically something that has large litters. From there it is up to the baby Changelings to assume the role of the litter. They do this by absorbing them into their bodies, becoming the creatures they absorb, albeit imperfectly. But that hardly matters because not only do they now smell and sound like the baby animal they subsumed, they have an extra special ability to affect the parent with a glamour/empathy cocktail. She will defend her tiny monsters with her life, and make sure they grow up to be strong little abominations who will set out into the world to find another creature to absorb and assume the role of.
If you have seen John Carpenter's "The Thing", you might get a little bit of an idea of how these creatures operate. They are not quite as nigh-indestructible as the alien beasts depicted in the movie, thankfully. Despite an infinitely pliable form they can be injured severely, and most of the traits they manifest are superficial. If a Changeling manifests proper wings after absorbing a hawk, it will be able to fly, but unless it also makes sure to copy that hawk's eyes precisely, a difficult task in of itself; it will be unable to fly as swiftly.
A changeling demon charms and unsuspecting traveller lost in the Bogs.
Changeling shifting is an imperfect process because it happens so rapidly. No Changeling can fully copy an organism perfectly, or at least, no young and inexperienced Changeling can. There is often a tell which is left over from the shift, but such tells can be smothered in their empathy magics. Mama hawk barely notices that baby has a beak where his left eye should be and is tearing up the rabbit she brought him with creepy talon-hands. Tells that might stand out to those not affected by the glamour may be eyes, hair (sometimes on an otherwise hairless animal) and hands/human like faces. A Changeling Demon that somehow never absorbed and morphed into anything, a blank slate, would resemble some small, pale Fae-hominid creature, though they are not Fae, it appears more a convergent evolution if anything.
Tells are somewhat useful however for Changelings to recognize each other. Despite their outward appearance, they cannot breed with other creatures, even ones they have somehow perfectly replicated, and depend on finding a Changeling mate first.
A common tell that almost always manifests are the erect plumes of hair that behave similarly to a bird's crest feathers. Each Changeling has a unique formation of these hairs, their version of a fingerprint.
As a Changeling continues to grow and change forms through absorption, it becomes more skilled at copying. Barring the tells which belie its true nature, an experienced Changeling will not have the misplaced parts and strange mishmash configurations of the young ones. They don't need to rely on their glamour and empathy as much at that point. Absorbing new forms is all in the service of attaining near-perfection in their next shape-shift.
A Changeling between forms can be very vulnerable however, as a bit of prep time is required before it can shake off the old form and attain the new. It has to take its next candidate by surprise, or by lulling it into a false sense of security with its magics. An animal that can defend itself in a panic when it realizes what is happening may mortally injure a Changeling, and having moulted its previous form leaving it in a weakened state, it will certainly die. A Changeling absorbing a victim seems to move like liquid or stretch like putty, but it is most definitely still one part.
A tendency to parasitize large litters due to the brood size results in many Changelings being animals rather than Elves, Fairy or otherwise. Thus, not many Changelings possess much in the way of sapient intelligence, indeed, such sagacity is a rare thing to find in a Changeling Demon... except in some very unfortunate cases. They will behave at the intelligence level of the beasts they have consumed (a small part will always be retained from previous forms) even if they do not always behave in the exact manner of said beasts. You might see a strange goose stalking a muskrat in the manner of a wolf, for example. That is not to say they are unintelligent, as many animals ARE rather intelligent, just in ways that are often inscrutable. A dog, which has no use for such nonsense vibrations, cannot appreciate music in the way that a bird, which has developed a mind for interpreting song, can. So a Changeling that was once a bird and is now a dog might actually respond to music by swaying to the tune instead of howling incessantly.
It is a bit unfortunate for a Changeling to absorb something of a higher intelligence. On some level, most Changelings realize this and wisely avoid it out of self preservation. Because the Changeling takes on the personality often times they lose their sense of self; they really believe they are the person they absorbed now. A Changeling does not understand why their loving family is trying to murder them all of the sudden.
It is rather a dead end for that Changeling's lineage when it cannot find a fellow Changeling appealing in the least unless it slips into something more sapient.
Sometimes the grim realization of what it did to become its current self is too much to bear. And as the Changeling who has become self-aware retains this form, they have lost their defenses and sometimes their will to continue.
Worse still, for those who do not grasp what they are, terrible accidents befall the ones they love. There is always that little urge in their mind to become a "better person"... and with many conflicting personalities being collected, it often drives them mad.
At any rate, Changeling Demons are not quite that common across Kellwood but are more abundant near the Bogs from which they emerged. However, like any creature, it is not out of the question for some to wander out into the larger world...