Changeling

A Changeling is a creature of fantasy, generally an offspring of an otherworldly species left to be raised by parents of a more mundane sort.

Folklore

Being folklore there are many variations, but they all follow the same pattern: a fairy kidnaps a child and replaces it with something known as a changeling. What the changeling is, how it behaves, how long the parent is stuck with it, and how you get rid of it sooner than that (if you should get rid of it), depends on the version. In some versions it's the fairy's offspring; in others, it's a piece of wood with glamour on it. Sometimes to get rid of it you throw it in the oven, other times you be abusive to it and the faerie switches the children again because you aren't taking good care of its child. (In practice, this had a nasty tendency of escalating to full-blown infanticide.) In Ireland it was common to dress up boys as girls up to the age of 12 or 13 because their fairies had a preference of kidnapping boys, and items like iron scissors would often be left where the child sleeps to ward off the fairy. The child that is kidnapped is also called a changeling. All mentions of changelings prior to this and the previous sentence refer to whatever the fairy left in the child's place.

World of Darkness

White Wolf's World of Darkness lines have a 'Changeling' game their original series, their LARP series, and the rebooted series.

Changeling: The Dreaming is for faeries that are born into mortal bodies and dealing with their dual natures. Characters live in two worlds simultaneously: the mundane one we're all familiar with, overlapped by a world we might call make-believe but is very real for them, and the two worlds can affect each other. There is always a tragedy with World of Darkness games; for Changelings, as they grow up the magical world becomes less and less real to them, until eventually they "grow up" and forget their natures as faeries. All Changelings eventually succumb to mundanity, living the rest of their lives locked out of their potential for magic. The different character classes are types of faerie: sidhe, troll, pooka, silkie, etc.

Changeling: The Lost is for mortals that were kidnapped by fae aliens, and managed to escape back to the real world but are forever scarred by the experience. Resuming their old lives is made difficult by time-dilation effects during their imprisonment, and possibly by a simulacrum or impostor that replaced them when they were kidnapped. Characters are a little alien themselves since they've spent most of their lives among fae, and have a hard time acting "normal." Character classes are damned complex with 39 Seemings+Kiths, 12 Courts, 9 optional Entitlements, 28 Contracts, and that's just what's in the core book.

Dungeons & Dragons

In Dungeons & Dragons, Changelings are a race introduced in the 3.5 edition campaign setting of Eberron as humanoids with a hint of doppelganger in their ancestry, which gives them limited shapeshifting abilities. In their debuting edition this takes the form of their supernatural signature racial feature is A Thousand Faces Minor Change Shape, which is similar to disguise self, but is not an illusion.

Warhammer 40,000

In Warhammer 40,000, changelings are Tzeentch's servants, and are total dicks. They're a special type of Horror daemon that have the ability to disguise themselves as anything, and use that ability to wreak all manner of mischief. However, because they have shifted forms so many times, most changelings have forgotten their original form. Only Tzeentch knows what each changeling originally looks like and commonly uses this as a bargaining tool to bind their loyalty to him.

One of the most famous hijinks with a changeling happened when one of them impersonated the Governor of the Hive World Raxos and threw it into total civil war, eventually conjuring a warp portal and having his other daemon buddies join in the fun. As usual, the Grey Knights were called in to contain the situation, which resulted in the total pacification of daemons in the uprising. However, the Grey Knights could not account for the changeling that started the war and assumed that it escaped in one of the evacuation shuttles that were taking refugees off-planet. So, in typical Inquisitorial fashion: the Grey Knights shot down all of the fleeing shuttles, killing thousands of civilians, in an effort to kill off the single changeling that MIGHT have boarded one of the shuttles. Overall, whether the changeling was there or not, Tzeentch was most likely pleased by the amount of "Just as Planned" it did to bloody an entire planet.

My Little Pony

In My Little Pony, changelings are insectoid shape shifters that feed on love and gain power from it. In their natural forms, changelings have gaunt bodies covered in holes and are equipped with insect like wings and a horn and they capture prey by wrapping them in some kind of webbing. They are led by a queen who is named "Chrysalis", who is the vanguard for an invasion by subjugating an area of conquest by infiltrating it from within and taking out its body of government to throw the rest of the place in disarray, which in the show's case was Canterlot's Captain of the Royal Guards and Princess Celestia.

After being beaten they attacked and ate a town full of kittens.

After their debut, the fans decided that when ever a pony was animated wrong (and it happens quite a lot [1]), appeared multiple times in one scene, or was acting in a manner that goes against the fanon, it was actually a changeling.

Despite how they are in the show many bronies have used the concept for pure nightmare fuel, something TG would be proud off if it came from ANY other source by my little pony.

Magic: The Gathering

A Static key word ability found on only on shape shifters. The ability makes the card it is on every creature type. This is a characteristic defining ability, so it applies no-matter what zone the card is in. [2]