Changeling

In 3.02 The Kids Are Alright, 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 mouth at the back of their necks. The changelings take their place in human homes as soon as they are formed, and feed off the mother until she wastes 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.

The true nature of the changelings - their discolored, slimy skin, hollow eyes and round mouth with many teeth - can be seen only in reflection.

Changelings in Lore

From Wikipedia:

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.