Swaplings

Not Doppelgangers, nor Changelings. These are faeries that are left in "exchange" for stolen infants, basically Fae cuckoos.
Created by magic, Swaplings take the shape of the child they replaced after being in its crib for about an hour. If it's discovered before the hour passes, the swapling vanishes and the baby returns. Condition of infant is up for a dice roll.
If the hour passes without the swapling being caught, it assumes the shape of the infant it's replaced, and everything pretty much sucks for everyone after that.
Swaplings are notorious for many reasons. Most run away from home (or are left out in the woods to figure shit out on their own, or executed, or tied up in the basement or any number of things). 
Their posture is often stooped, their speech is slurred, their heads loll and their limbs move erratically. This is actually because the natural shape of a swapling is quite small, and it's trying to steer a much larger creature with the wrong interoception/prioperception. It's also usually really fucking hard to think for the poor thing, the sensory shit doesn't suit this body either.
Swaplings are not naturally malicious, but growing up like this is difficult for them and they often lash out in anger and it gets them a bad reputation for rage. It doesn't help that those who use corporal punishment don't realize they're showing the Swaplings that violence to get your way is how it works, and swaplings are made of magic.
They're conscious, mostly, but not very intelligent. They do respond to speech and language and can usually learn to speak if someone is patient enough to teach them. Simpler languages are easier for them, and most use body languages and noises just fine. Most do not learn to read or write. 
They are perfectly capable of affection, and will often pick a person they like best. The person they attach to may or may not appreciate this, as their affection varies between a merry wave and running over to humping their leg.
The sole reason they exist are to count as an "Exchange" for the stolen infant. It's a great little loophole for stealing babies if you need one. Creating them is a bitch and a half, and it's usually easier to bargain one away from its parents, but this is for emergencies like spell components, leverage, or whatever.