Demons

"If you were to ask a demon how it came to be —assuming you found one capable of holding the words in mock comprehension (pony speech is all too limiting and far too complex), and who had a willingness (and ability) to stave off its hunger in favor of conversation — they would have no answer to give." 
"If you were to ask a demon how it came to be, assuming you are immune to the brutal pulls of nature (to a tsunami, a wildfire, a death in the family), it is already too late for you. And for that, I am sorry." - Demonologist, Eastern Dawnstar IV
== What We Know ==
=== Territory and the Border ===
Demons make their home outside the Equestrian borders. For whatever reason, divine or otherwise, demons are not able to make their way into Equestria. Outliers exist, the demon who ravaged the land now known as the Torrid Remains was able to breach that barrier, but the "how" and "why" are unknown. The border itself is known to ponies as well, described as an inescapable feeling of dread and panic, intensifying the further you push past it. There is no hard line between Equestria and the demon lands, just an understanding of where one belongs and the other does not. 
=== Biology and Abilities ===
They need not for sustenance to survive, no amount of "going without" can kill them. While they can't starve to death, they do hunger. They hunger and deeply, something primal within them starves, gnawing away at them. They feed on the flesh and the souls of ponies brave, and dense enough to wander from Equestria's border. They may prefer one over the other. 
It is currently unknown what can kill them. It can obviously be done, it's been done before. The Torrid Remains, ravaged and volatile as they are, are proof of it. Unless something divine such as that brings them to an end, they will live on for an eternity. They are as close to immortality as one can be, with exception of the Goddesses 
They can possess a wide range of deadly traits. Some may fashion rows of jagged, gnarled teeth and serrated claws that can rip into someone with ease. Others may harbor toxic, gaseous breath or acidic spit that burns through even the toughest metals. All of these serve no evolutionary purpose, they only serve the demon's bloodlust. These traits are most often weaponized against their fellow demons but it is not unheard of for a demon to have traits that actively cause themselves pain. 
Those who favor feasting on souls may approach consumption in other ways. Getting in the mind of their victims through a connection to the soul, done by any number of sadistic and magical means. How they worm their way into one's soul is still unknown, some manage it through conversation but that's the only method that's traceable. They drain your magic and vitality from within you as they take your soul rather than consuming it with your body. Victims of these demons may even be taken over, used as a vessel to attempt to bypass the border. This hasn't worked yet, thank the Goddesses. 
== What We Don't ==
=== Territory and the Border ===
We do not know what keeps them at bay. We know not of the magic that shields us. We know not of what c̶o̶u̶l̶d̶ will bring the barrier down. 
=== Biology and Abilities ===
They don't live. Watch them, would you call that living? Would you even call that survival? Surely not. They ravage the world they inhabit, cannibalizing themselves and festering just outside our borders. Why do they persist?
=== Mortality and Morality ===
They will outlast you. They cannot care. It is beyond and below them. We are beyond and below them. Us ponies, we are vast yet miniscule. They cannot grasp what we know, what we hold dear. They will not mourn us when they are done. Pray that the Goddesses take us in when they are."Death does not come easy for demons. Not by a pony’s will, nor by the creatures own. A demon is a beast too familiar with hunger to die. As long as they live, they will hunger, and as long as they hunger, they will live. Even the natural order seems avoidant to them. They drag on, starving, bloodied, torn to unrecognizability. A sickening persistence pushes them onward. We do not know if they suffer."
- Demonologist, Eastern Dawnstar IV's Partner 
File:Demon 1.png|The wall crawling acid spitter responsible for the Torrid Remains
File:Demon 2.jpg|Knuckle walking demon
File:Demon 3.jpg|Demon with false plant lure
File:Demon 4.png|Demon with lame back legs and feet that have been mangled to raw bone