A shape-shifting quadruped species, feeding on the emotion of love, which is said to have successfully infiltrated the Equestrian government in the Golden Age, with the goal of maximizing love production for their own nourishment. The species was apparently greatly reduced in numbers during the Catastrophe, as the result of their central hive-mind being destroyed. In modern times, the open and peaceful existence of changelings is known; it is generally assumed that there are also a number of clandestine changelings, but reliable information on that topic is scarce.
After PRPG2
Some time after the end of the contiguous sessions of PRPG2, the Elements were used on the changelings of Equus, and the Elements extended the effect to cover the entire universe, including any changelings within it currently known or unknown to the players. All changelings and Crystal Forest were affected, and all but those who had diverged so far that they were simply turned into something else were, after where necessary being healed enough that they were capable of choosing, informed to the necessary extent of the state of the world (this applying mostly to those changelings who had been in the Crystal Forest) and given a choice: be fully healed and exiled to a place with abundant love, but no way to return, or be allowed to stay while accepting more drastic changes. All of this happened near-instantly, with magic granting them a brief time to confer amongst themselves (and, where harmonically appropriate, with relevant non-changelings) and choose but no chance to even attempt to escape or use their powers harmfully (or be harmed) during the in-between state. The Elements did not treat all changelings as a single group, instead evaluating the changelings they found in the universe and assigning them to different groups based on their characteristics, such as their relative locations and their preexisting connections. This had an effect mostly on the telomere (explained below) given out, with different groups being given different, incompatible ones of potentially different sizes, and the information the changelings were provided with to help make their decision, with generally little or nothing being given about even the existence of other groups.
The Elements also placed a persistent spell upon the universe such that any changeling entering from outside (a contingency for the exile somehow failing or, more likely, some unknown preexisting colony of changelings in another world) or traveling in time from before the spell went into effect, or in any other way having evaded the spell and now entering the area of its effect, would be given the choice to accept harmonization by the standard terms or an appropriate approximation thereof, be thrown back to where they came from, or be randomly sent somewhere else in the multiverse.
The harmonization changes are as follows:
1: Harmonized "rainbow" changelings have been transformed, their base forms altered to something more colorful and friendly-looking (they are still capable of assuming their old forms, but changeling-transformation-dispelling magic will now revert them to the new ones), and given an empathetic nature "in the image" of ponies; their regard for ponies (and similarly sentient beings) is now like the regard of ponies for other ponies. Just as ponies can be more loyal to a particular group of ponies than another, so too can a particular changeling side with one group of ponies over another group of ponies (and/or changelings). Changelings have free will of association and preference, and can dislike and even fight against groups of ponies in the same way a pony could, but likewise, the idea of killing large numbers of ponies for a small and unnecessary gain is as repellent to them as it would be to most sane ponies. This prevents things like, eg., all changelings uniting to conquer the world and "save" ponies from themselves.
2: Harmonized changelings produce enough love for their own survival and day-to-day magic use. Like the magical reserves of a pony, this love slowly regenerates after being used. A harmonized changeling can therefore usually do average-pony levitation, flight, and so forth without worrying about running out of love, but higher magic use could outpace this natural generation and require an external source, a resting period of reduced love use, or both.
3: Harmonized changelings are immune to the Crystal Forest "disease", and if they were part of the Crystal Forest or have otherwise obtained the ability, can form themselves (or parts of themselves) into the substance of the Crystal Forest that produces love from sunlight without having to take it from another sapient creature.
4: Harmonized changelings may reproduce, but their total population is capped by a hard-limiting mechanism. Part of the harmonized changeling soul acts like a collective telomere shared and evenly distributed among all changelings in the group. During reproduction, the new changeling soul must receive at least a certain minimum share of this soul-telomere in order to be viable. This means that once a certain maximum number of changelings exist, trying to produce a new one will result in its parent(s) not being able to provide enough telomere and reproduction will fail. When an existing changeling dies and brings the population back below the cap (assuming it was at the cap before), the quantity of telomere the deceased changeling possessed is redistributed back among remaining changelings, which frees up enough to produce one new replacement changeling.
Equus
The Elements considered Equus and its local space to be a single, in the harmonization sense, group. A majority of the changelings who had been trapped as Crystal Forest chose exile, but a great many out of that large population did not, and the majority of changelings who had escaped becoming Crystal Forest chose to stay and be transformed. Although the most grievously damaged changelings were healed or otherwise dealt with and the ability to group into hives was restored, there still remained quite a variety of different changelings, with different personalities, and different changeling social groups. Some of these groups, even some hives, lost members or fell apart completely in the changelings' efforts to adjust and their attempts to work out their new relationships with the non-changeling peoples of the world, while other groups and individuals came together to form larger arrangements of various forms. Modern, PRPG3-era Equus still has its varied changelings scattered around the world, most with at least some level of open integration with the non-changeling world, and while relations are sometimes rocky, for the most part the rainbow changelings are just another people of Equus.
The largest single known modern concentration of changelings on Equus is in and under New Trenchtown, after many of the rainbow changelings, particularly ex-Crystal-Forest ones, were drawn to Trenchtown or later New Trenchtown's hive for its relationship with the Elements and its relatively intact survival, and the city is the closest thing the planet has to a changeling "capital"; this basically only means having a set place to meet for anything considered to be of major concern to a large portion of the species, though, such as the occasional argument over telomere allotment, with the planet's changelings being politically and ideologically disunited.
Exile
Those changelings who chose exile were, as the Elements said when offering the choice, fully healed, then banished to a far-distant dimension devoid of other life. There, by the physics of that place, they could live their lives without preying on others, but neither were there any beings for them to be a threat to. There was nothing there that could threaten them, either, allowing them to live in peace if they so chose, and sturdy barriers were placed around the dimension against escape or incursion. In particular, the Elements took great care to ensure that the exiled changelings could not get back to Equus's universe.
The Colony Ships
The two sister ships of the Harmony that were able to escape Equus are out-of-universe known to have survived their flight from the planet, though their locations are unknown and contact has not yet as of PRPG3 been reestablished. Due to the nature of the magic they used to escape, they could be reappear more or less anywhere in the universe at more or less any time after they departed (it is, however, defined that they reappeared at some point prior to the use of the Elements on the changelings following PRPG2). Little has been defined about them overall, but it was desirable that some information be given here on the fates of the changelings aboard.
Luna's Changelings
Luna, while upset at the changelings' actions, was wiling to hear them out and negotiate, in large part due to her past with them and role as their creator. Not wanting to destroy them and knowing how useful they could be to her ponies but also unable to truly trust them and unwilling to demand that her ponies do so, she worked with them to alter their nature, as Chrysalis had long ago, this time to give them a ponylike inherent regard for non-changeling sapient life. This change made, the changelings continued among the people under Luna, beginning to repair their relationship with her and them. When the Elements were used on the changelings, all of those in this group chose to stay.
Celestia's Changelings
Celestia quickly killed all of the changelings on her ship, her past experiences with them being overall far more negative than Luna's. There were none left when the Elements were used on the changelings.