Changelings

Characteristics
They have been talked about throughout history as being the replacements for human children who are stolen away by monsters. The children are usually taken underground, and fed on by the mother changeling. The changeling that replaces the child looks the same as the original child, but is evil, and otherwise acts differently. They feed off the mother of the child they have taken the form of. They are very protective of their food source and will let nothing separate them from it and will kill anyone who gets in their way. The mothers get fed on gradually and will last a few months before death, at which point the changelings move on to the next neighbourhood.

Although they act somewhat like children, the young are cold and do not seem right. As such, some mothers notice that it is not their child.

Physical Appearance
Changelings true form is discolored, slimy skin that appear to be moulting or covered with varicosities, hollow eyes and round mouth with many teeth. A changelings true form can be seen only in reflection in a mirror or other reflective surface.

Season 3
Sam and Dean come across changelings in Cicero, Indiana. The activities of the changelings are orchestrated by a "mother" changeling, who disguises herself as an adult woman. The changelings steal children in the neighborhood from their beds at night and then take the real children's places in their family homes. The real children are kept alive, in cages, in the basement of a house abandoned mid-construction, for the "mother" changeling to feed off.

Meanwhile, the child changelings feed off of the human mothers' synovial fluid by suckling with their round, many-toothed mouths at the back of their necks. The changelings take their place in human homes as soon as they are formed, and feed off the mothers until they waste away. Any other humans who come between the changeling child and its prey are murdered. The mother changeling is finally killed by fire (Sam and Dean use a flamethrower). When the mother is killed, the children are destroyed also.

Powers and Abilities

 * Super strength - They have superior strength to humans, however they are quite weak compared to most monsters. Young changeling are not strong enough to break through a door, while Dean was able to fight a full grown one.
 * Lamprey-like mouth - They have sharp teeth that allow them to feed on humans, and a lamprey-like mouth.
 * Shapeshifting - They can appear human.
 * Super agility - Are able to climb walls to snatch children.
 * Invulnerability - Can only be killed by fire, but are not completely invulnerable, and can still be apparently hurt by blunt force.

Vulnerabilities

 * Fire - Setting a changeling on fire will kill it.
 * Family - Killing a mother changeling will cause its children to die instantly.
 * Mirrors - A changeling's true form is revealed in a mirror.

Known Changelings

 * Mother Changeling (deceased)
 * Katie (Changeling) (deceased)
 * Ben (Changeling) (deceased)
 * Changeling Children (deceased)

Lore
In European folklore and folk belief, a changeling is the offspring of a fairy, troll, elf, or other legendary creature that has been secretly left in the place of a human child. The apparent changeling could also be a stock, a glamorized piece of wood that would soon appear to grow sick and die.

In the past, European countries used to believe that a child who exhibited symptoms of autism had in fact been replaced with a changeling. As a result, some parents left these "fake" children to die in the woods.

Appearances

 * Season 3
 * The Kids Are Alright
 * Season 9
 * The Purge (Mentioned only)