Changelings

Often the otherworld child is described as having an insatiable appetite, or alternating between tiny and huge, unpredictably, or suddenly becoming a wizened but small child.

Theodore Sturgeon wrote a odd / weird story about a changeling called "Brat."

I don't recall them being 'physically terrifying', not horns'n'fangs'n'giant claws, more sort of shrunken and wizened, or the creepiness of age-inappropriate stuff, like talking, or walking very early, or staring intently at stuff (a popular way to detect changelings was to do something bizarre, like brew tea in an eggshell, which would cause the changeling to say something like 'Old as I am, I've never seen that before!').
In many versions, it's an elderly fairy playing at being an infant (presumably for a soft bed and steady meals), so a shrunken, wrinkled little body, but immensely old and knowing.
I get the impression that the usual changeling behavior is more something it's doing for lulz and for kicks. It is, after all, boring to just lie there in that nice warm cot. (You might get some horror mileage out of the 'baby' waiting until 'the coast is clear', everyone's asleep or out, and then hopping out of it's crib to grab a snack or watch TV, or just snoop around or mess with stuff.)

I think the real 'horror' is the insatiability, the constant neediness, a child who never gives the family a moment's peace. Like a colicky child who never stops crying, who eats everything given to it, but is never full,
who wails for some unknown thing. A nursing baby who wants the breast right now, but won't actually suck; a baby who throws all it's toys out of the crib, and waits for you to return them, so it can do it again. Now all babies do that, but imagine that you can feel it's doing it intentionally, can sense it's beady little eyes watching you, waiting for you to start some task so it can do it again, and again.
People say it's just 'teething pains' but the baby never gets any teeth. (Or, just one, so it can bite down on the breast.) And, there's no sign that the baby is going to grow up, or grow out of it's behavior.

That's it's effect on families, too, the constant crying, setting teeth on edge; the way that everything revolves around the 'baby', the constant efforts to appease it, to get it to JUST SHUT UP FOR ONE DAMN MINUTE!
Of course, if this is a traditional changeling story, where the creature has replaced the family's own baby, there's the horror of knowing that your own baby is out there - somewhere. I have read one story where the people with the changeling have managed to trade their baby back. In that case, it wasn't an old fairy, but a fairy infant, and it's mother was glad to get it back.

And, do read 'Brat', as mentioned by AW Admin.

Note that the changeling myth was still strong enough in the early 1800s that there were newspaper accounts of babies being inadvertently hurt and killed by "changeling tests," typically involving fire, hot pans or water.

A lot of the so-called changeling children were actually infants with developmental disorders like Down's or Progeria or even something as benign as a port wine birthmark that might give them an appearance that wasn't expected. That was often all it took for a parent to be "horrified."

frimble3 said:

That's it's effect on families, too, the constant crying, setting teeth on edge; the way that everything revolves around the 'baby', the constant efforts to appease it, to get it to JUST SHUT UP FOR ONE DAMN MINUTE!

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As I was reading your post, a horrible thought crossed my mind.

What if the "changeling," was really just a normal, albeit colicky child? However, as a lonely mother struggles with new motherhood, her undiagnosed post-partum depression sparks an increasing sanity slippage that makes her believe that her baby is a changeling.

Write the story from the perspective of the mother as an increasingly unreliable narrator, and you could have a pretty terrifying psychological horror story.

tooloo said:

As I was reading your post, a horrible thought crossed my mind.

What if the "changeling," was really just a normal, albeit colicky child? However, as a lonely mother struggles with new motherhood, her undiagnosed post-partum depression sparks an increasing sanity slippage that makes her believe that her baby is a changeling.

Write the story from the perspective of the mother as an increasingly unreliable narrator, and you could have a pretty terrifying psychological horror story.

Click to expand...

And it's situations like that, played out in isolated farmhouses, no other resources or perspectives, that were probably the origins of changeling stories. (Especially if they lead to infanticide - of course it was a supernatural being that magically flew away - no dead baby here, no,no,no!)