From Strange Lands: A Field Guide to the Celtic and British Otherworld by Andy Paciorek

A mother had her child taken from the cradle by elves. In its place they laid a changeling with a thick head and staring eyes who would do nothing but eat and drink. In distress she went to a neighbor and asked for advice. The neighbor told her to carry the changeling into the kitchen, set it on the hearth, make a fire, and boil water in two eggshells. That should make the changeling laugh, and if he laughs it will be all over with him. The woman did everything just as her neighbor said. When she placed the eggshells filled with water over the fire, the blockhead said:

Nun bin ich so alt    Now I am as old
Wie der Westerwald,    As the Wester Wood,
Und hab nicht gesehen,    But have never seen anyone
Daß jemand in Schalen kocht.    Cooking in shells!

And he began laughing about it. When he laughed, a band of little elves suddenly appeared. They brought the rightful child, set it on the hearth, and took the changeling away.

from “The Elves,” Grimm’s Household Stories, 1812
Story collected from Gretchen Wild (1787-1819).
Translated by D.L. Ashliman.
Andy Paciorek: Strange Lands: A Field Guide to the Celtic and British Otherworld

Illustration by PJ Lynch.

Changelings are creatures of Faerie who have surreptitiously been left in place of not-yet-baptized human infants, who in turn are spirited away to live in Faerie. There are many theories as to what the nature of the Changeling is, and what purpose is served by this secret swap—one, which the story above is illustrative of, is that the Changeling is an old male elf who is basically on vacation, taking advantage of unwitting human hosts, and being pampered and fed all day long. As in the story, once discovered as impostors, these types of changelings tend to give up the act and bring the human infant back while themselves returning to the Fair Lands. If they can be made to laugh or speak, which is most easily done by surprising them, they will consider their little holiday over and quickly depart.

Changelings in modern stories:

In Supernatural, Changelings are horrid creatures with lamprey-like mouths who feed on the synovial fluid of adults, and in particular, their human surrogate mothers. As with classic changelings, the creatures in Supernatural have a sort of glamour that disguises their true nature. They can still be seen in their undisguised form in mirrors, however. The stolen children who these changelings replace are locked up and used to feed the changeling mother. Killing the mother of this monster will also immediately kill all of her offspring. (See S3E02, “The Kids Are All Right.”)

In My Little Pony: Friendship is Magic, Changelings are malevolent creatures that have a pony-like shape, but insectiod construction. They have hard beetle-like carapaces, insect wings, and fangs, among other bug-like characteristics. MLP:FiM Changelings have shape-shifting abilities, and feed off of love, which is why they are drawn to Equestria. (See S2E26 for the Changelings’ first appearance.)

In the Star Trek universe, Changelings are a race of lifeforms from the Gamma quadrant, clear and gelatinous in their natural state, but able to take on humanoid or other solid forms for several hours at a time. They do not eat, and cannot smell. “The Changeling” was also the title of an original series episode of Star Trek (S2E08), upon which parts of the first Star Trek movie were based.