Changelings are creatures from the second season of MLP: Frendship is Magic. As in the show, they can take the form of other ponies, disguising themselves as the Mane 6 in this case. Once revealed, they are similar to parasprites, being obstacles that need to be cleared by using the appropriate type of Element Shard. Unlike parasprites, one changeling will take up 5 of the same type of elemental shards to defeat. Each one stands in the center of a little crater which acts as an impediment to building, which will only disappear after the changeling has been defeated.
Note: On some devices the crater will not be properly removed after the changeling's defeat, or the changeling will just stand there without the aura. It should be gone on your next visit to Canterlot, however.
Note: Changelings can sometimes land on the bottom and side borders of the terrain/screen, which can make it difficult to tap them. Zooming in helps a little. It also helps to create a anti-landing zone barrier of sorts with small objects like trees along the left, right, and bottom edges of the terrain or screen, particularly if you have cleared all the zones in Canterlot. Simply putting down Dirth Paths or Cobblestone Paths will not work, as Changelings can land on them.
Note: Changelings sometimes land behind buildings or trees to the point where you can barely spot them. Try to keep track of how many you've uncovered and where they've landed to make sure you get them all.
Second, they can show up in disguise as any member of the Mane 6, but typically prefer to be disguised as Twilight Sparkle. If you go to Canterlot you may see several copies of Mane 6 ponies running around. Only one of each of them is the real one. Simply tap on each one and if they respond as usual they are real. You will also see at the top right corner, a picture will pop up and show how many stars they have, if they are real. If they are a changeling they will reveal themselves as such, jump and disappear in a column of coloured light and land somewhere in your clear zones creating its crater.
Even when disguised as a pony you can often tell which copies are changelings with careful observation of their behavior. Disguised changelings will wander anywhere on the map, even well into the shadowed zones. They are also not as social as regular ponies in that they won't chat or play with a group of others and are usually found alone (they'll stop if blocked and other ponies might stop for them so seeing a pony in a crowd isn't a reliable way to weed out changelings). They won't react to new ponies being welcomed. One last method applies specifically to Twilight Sparkle. If you have advanced to the point where Twilight has become a princess she will have wings and a crown -- but disguised changelings will still appear to be regular unicorns.
To clear changelings tap on them 5 times. They have a 'health bar' with 5 lines which appears when you tap them, representing the 5 shards needed to defeat them. Each time you tap on them, you use another of the same type of shard to lower their health.
When clearing changelings, you can obtain bits as a reward on the removal of each bar. There is also a small chance that a single gem may pop out when you defeat it. In the Windows version you obtain wheels to play the Crystal Mine Game, unlike the rest of the earned wheels, one wheel from the changelings allows you to play. Bits are still occasionally rewarded.
As a part of the quest series in Canterlot there are several quests that will require you to uncover and/or defeat changelings.
In addition near to the end of the questline, you will have to defeat Queen Chrysalis by clearing 6 waves of changelings, each consisting of 4 changelings of the same type around the Queen and possible optional changelings in other parts of the city. All changelings around the Queen in each wave must be cleared completely before the next one spawns.
However, unlike in the series, defeating her will not stop changelings from spawning. Just like parasprites, changelings will continue to spawn in their various forms.
Types of Changelings
There are 6 types of changelings, matching each of the Elements of Harmony. They can be distinguished by the colors surrounding the rim of the crater. (Note the Windows 8.1 version does not display the auras, thus there is no way to tell which element is required until tapping on them. The Windows RT version does correctly display the auras.)