Exactly what Changelings are and how they operate is something that few people here on the forums have a concrete consensus on, at least in my experience in the Roleplay section. This is additionally complicated by the introduction of 'Reformed' Changelings, which I will discuss at the end of this post.

I personally derive most of my changeling functionality from the FiMFiction story, A Novel Tale, but I have made some modifications. For the sake of clarity, I will write here assuming that you have not read the story.

If there is anything below that is confusing, or an aspect that I missed, please let me know in a comment or message so I can improve this resource.

Biology

Most creatures on Equis can be described as biological creatures with magical abilities. Changelings are an exception to this, being better described as magical entities with biological components. Changelings are, in essence, primarily composed of magic. This allows them to change their physical form and outward appearance at will, but also gives them a need to consume energy in order to sustain themselves. A Changeling has a physical body covered in chitinous plates composing an exoskeleton, making them especially lightweight yet durable, and a magical barrier that covers their body. The barrier is malleable, and is what allows them to change their appearance, as well as protecting their body from being exposed to air. A changeling's body is highly flammable, and will combust whenever exposed to air, which is why they are engulfed in fire when they change disguise, as the barrier temporarily disappears as it changes. This barrier is always present, even when undisguised, except when a changeling dies or is entirely drained of energy.

When a changeling dies, the barrier disappears and the body is burned. This leaves behind a pile of ash, called Changeling Ash, whish is itself highly flammable and useful as a fire starter, often mixed with fire salts or used as a pyromancy ingredient.

Reproduction

There are a few ways whereby changelings can reproduce, but all methods require a Queen. Changelings, biologically speaking, are genderless. While they are capable of properly disguising as any gender, they are incapable of impregnating or becoming pregnant. The technical exception to this is a Queen, who is the only type of changeling capable of producing ovum, and a King, who is a transformed pony and retains their gender from their original form.

A Changeling Queen can make new changelings in one of two ways. The preferred way is via cocoon, but the queen can also lay eggs directly if a cocoon is unavailable or in emergency situations. For the cocoon option, a cocoon is made and filled with emotion gel, which is described in greater detail below, and this gel is charged with the primary emotions of the hive, with some others mixed in. The exact mix is rarely consistent, and is attributed with being the driving factor for the predispositions of the changelings grown within. Once the cocoon is prepared, the Queen deposits a number of eggs inside depending on the size of the cocoon, typically 6-12, though larger cocoons can be made to hold upwards of 40 at a time. The gel in the cocoon is periodically recharged as the eggs grow, and after a few months of gestation the eggs hatch and the young changelings, referred to as foal-lings, emerge. These foal-lings will continue to grow and develop over the next few years, being educated by their hive and working different jobs inside the hive both to help the hive grow and to build skills that they'll use when they go out into the world.

In the case where a cocoon cannot be made, the Queen can lay eggs directly, like any other egg laying species, and these eggs are then covered with emotion gel which is charged and recharged the same way as above. When done this way, typically only 1-3 eggs are laid at a time.

Physiology

A Changeling has the same body shape as an equine, which is to say quadrupedal, and has wings and a horn. Their bodies are almost always a shade of dark grey, similar to thestrals, with accent colors on their backplate, tail, mane, wings, and eyes, representing the primary emotions of their hive. In this way, changelings from different hives can be visually distinguished, which changelings from the same hive will look similar.

That is not to say that all changelings from a hive will look the same. In their undisguised form, the arrangement and size of the holes in their legs is mostly unique. Further, the relative sizes of their legs, wings, and horns, can vary drastically between members of a hive, though are often similar among changelings hatched from the same cocoon. These are referred to as Predispositions, and often influence the type of disguises that changelings from that cocoon will choose to wear and how they will view themselves. 

Diet

While it is accurate to say that changelings feed on emotion, it is more accurate to say that they feed off of the emotional energy given off by those around them. In emergency situations this energy can be drained directly, but this can be damaging to the one being drained and is less effective of a food source for the changeling.

The optimal food source for a changeling is emotion gel charged with the primary emotions of their hive. Emotion gel is typically made in the hive in large vats utilizing boiling mineral water, usually sourced from an underground hot spring, and changeling bile which they produce inside their bodies. This substance, also called changeling spit, is highly viscous and can be hardened with magic, making it a very good building material or restraint method.

Once the neutral gel is made, which will often be the same color as the changelings who made it, it is barreled and shipped out to the Harvester changelings who charge it with the emotional energy they have collected rom the creatures around them. The charged gel is sent back to the hive to then be distributed to all of the changelings who need it, as well as to feed the changelings currently living in the hive.

A changeling who is out scouting can also make their own gel with any water, though mineral water is preferred, so that they can digest the emotions they are collecting without having to return often or carry a lot of food with them.

Interactions

Optimally, changelings interact with other species in order to cultivate their desired emotions in those they come into contact with. In a way, they act like bees where the other species are flowers. The changelings strive to ensure that those around them are healthy and capable of producing their desired emotions naturally, encouraging them to experience them. For example, a hive with a primary focus on awe would likely be involved in the tourism industry, building things that would generate awe and encouraging those around them to visit and see it. 

There are then two types of changelings involved in the energy collection process, the Cultivators, and the Harvesters. A Cultivator's job is to do whatever is necessary to inspire the desired emotions. This will often involve engaging with other creatures directly. A Harvester's job is to discretely collect the cultivated emotions. This process can depend on what the desired emotions are, some take time to fully cultivate, some are quite fleeting and have to be collected quickly, and some will last a good long while after they have been cultivated. Usually a Harvester will take a job that allows them to interact with many creatures for short durations with minimal notice, like a mail carrier, waiter, or checkout attendant. Ideally, the cultivated creature will never even know that their excess emotional energy has been collected, and will generally be better off for having, unknowingly, interacted with the local changelings.

In addition to their jobs in the hive, changelings will often seek fulfilling relationships with the creatures around them both out of social need and to better integrate themselves in the local community.

Reformed Changelings

Many changelings view 'Reformed' changelings as a perversion of the natural way, this is due to their lack of familiarity with this development and the unfortunate connotations of the name 'reformed'. Many consider it a sort of social reformation, like how one would reform a convict, implying that changeling nature was originally evil and that reformed changelings are thereby good.

In truth, reformed changelings are such in a more physiological sense. They have been literally re-formed into an entirely new species more akin to ponies than changelings. Where changelings are magical creatures with biological components, reformed changelings are biological creatures with magical abilities. Unlike traditional changelings, a reformed changeling:

• Does not have a magical barrier surrounding their body when undisguised
• Does not burn when exposed to oxygen or killed
• Does not need to collect emotional energy in excess of what they produce for themselves
• Has a biological gender, and reproduces using traditional methods
• Does not always have both wings and a horn, and may have neither
• Has reduced ability to adjust their size/shape while disguising