Changelings

One of the world's most significant threats.
Changelings were not discovered until Celestia and Luna were tapped out of the power Caleph and Venus provided them with to banish the kangaroo and sentient carnivores.

Overview
Changelings are harsh, violent natives of The Badlands. They are sentient beings that possess an equine shape, yet strongly resemble insects with their beady eyes, tough skin, and thin wings. All changelings are carniverous and can manipulate ley in the same way unicorns can, though their ability to manipulate ley to the extent unicorns can is limited.

Their natural died whatever wildlife they can catch and, more disturbingly, each other. They have another, strange way to sustain themselves, however.

Emotional Power
Changelings can "feed" off of the emotions of other species and off of other changeling hives. No one has been able to study how that is possible other than the changelings themselves, so knowledge on this subject is limited.

Each hive has its own "emotional frequency", so hive members of the same hive cannot feed off of each other's' emotions.

In addition to providing a secondary source of power to cast spells with, feeding off emotion of another being significantly enhances a changeling's ability to process and manipulate ley, allowing them to cast more powerful spells. At least, ones that use ley. This means that if a changeling were to feed off of emotion long enough, their ley-magic power could rival the power of a unicorn.

Culture
It is believed that there are hundreds of hives scattered all over the badlands. Because of their numbers and violent nature, it seems that the only thing that has prevented them from constantly warring with the other species and other nations is the fact that they fight amongst themselves.

They work with metals, like the rest of the world, though to a lesser extent. To the best of pony's knowledge in the first several years after discovery, they do not make firearms. They still make swords, blades, spears, and armor. However, later on, it was discovered that changelings do produce firearms, they just rarely use them.

Presently, changeling tactics consist of very, very powerful artillery and swarming with melee tactics.

Species, Breeds
Changelings, while they all share the facts above, are divided into different species.

Worker
This type of changeling has long, sharp, scythe-like legs. Their wings are far thicker than the other species', and when not in use, are covered by a thick shell that opens whenever they extend their wings. They can dig out tunnels incredibly fast. This breed is far less intelligent than the others - they have to be trained and kept in pens or cages. They can be controlled en masse by a king, queen, or tyrant. This species cannot shapeshift.

Warrior
This breed has a much thicker hide than other changelings and tend to be larger. They have large fangs and much sharper teeth than the other breeds, and a Warrior's muscles are easier to tone.

Wraith
This is the least common type of lesser changeling - capable of shapeshifting into another creature for much, much longer than the other types. While the other breeds of changelings can stay in another form for several weeks, this breed can decieve others for several years. On top of that, they can assume up to five month's worth of memories of those they take the shape of if - they can get to them. Wraiths are the most effecient at gathering power from emotion. Before Chrysalis's actual invasion, these are the changelings that helped her infiltration be so successful.

((Wraith definition - A ghost or ghostlike image of someone, esp. one seen shortly before or after their death.))

Tryrant
This is a minor leadership breed. Tyrants can cast spells and manipulte ley with such ability to rival an alicorn's. Tyrants can create a telepathic hive mind, an ability specific to changelings. Every changeling that enters this hive mind will be able to hear the thoughts their leader projects. A tyrant can force changelings into a hive mind.

Each hive's tyrants have their own "hive mind frequency," as well. No changelings can become a part of the hive mind of changelings from another hive. Changelings can only be listening to a single tyrant at a time, though. Tyrants rarely can have more than about one thousand changelings per hive mind.

King/Queen
These are, as one might imagine, the leaders of hives. They can indefinitely assume the shape of another being. Kings and Queens have no limit to how many changelings they can have in a hive mind at once. One major limitation of the monarch breed, however, is that their horns cannot process ley. They rely entirely on feeding off of the emotion of other species to be able to cast any sort of magic. They can "collect" power from their underlings, but it takes thousands of changelings to contribute their own gathered emotional power for a King or Queen to be able to cast any sort of major spell.

Chrysalis is unique. At will, she can assimilate all the memories of those she can reach. She also appears to have a much, much longer lifespan than any other changeling. She has already lived for more than 800 years, to the best of Equestria's knowledge, while most changelings do not live past 200.