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Littlepip's party facing off against several hellhounds under Enclave control.
~ 
Felixattchar
Hellhounds are an intelligent species of sentient, bipedal canines. They have powerful claws which they can use to tunnel through the earth and solid concrete. They are the highly evolved descendants of diamond dogs, having been mutated over two centuries by megaspell radiation.
Background
Pre-War
- Main article: Diamond_Dogs on the My Little Pony: Friendship is Magic Wiki
Prior to the Great War, diamond dogs resided mostly in tunnels under Splendid Valley, where they maintained a tribally-divided civilization. They mainly kept to themselves and maintained neutral, if cool, relations with the pony nation of Equestria, whose lands they inhabited.
During the War
During the Great War, Equestria evicted the diamond dogs from Splendid Valley with the intent of using the valley to dump megaspell waste from the Ministry of Arcane Sciences' I.M.P. project in Maripony. They were relocated near Ponyville, where they were tasked by the Equestrian government to mine the nearby area for gemstones. It is unknown if diamond dogs took any other part in the war besides mining gems.
A group under orders to be relocated by Princess Luna ended up murdering a pony in self-defense, furthering the animosity between the two species.
Post-War
After the war, the vast majority of diamond dogs instinctively returned to Splendid Valley, only to suffer from the radioactive fallout and be mutated into the modern hellhounds. Due to Equestria's treatment of diamond dogs as second-class citizens during the war, hellhounds maintain a high degree of animosity and hatred towards ponykind, and typically kill ponies on sight.
Modern Day
During the Wasteland Conflict, the many hellhounds living under Splendid Valley were killed when Littlepip detonated a Balefire bomb under Maripony that killed the Goddess. The survivors took refuge in the Everfree Forest, where they were harassed by Red Eye's Army and the Enclave pegasi. A large number of hellhounds were captured by Enclave and had mind control devices placed on their heads, forcing them into servitude, and this did not end until Littlepip's eventual victory over the Enclave.
After the end of the Wasteland Conflict and the establishment of the New Canterlot Republic, a small number of hellhounds decided to join the NCR in hopes of restoring civilization to the wasteland once again. The vast majority, however, remained detached and independent, still harboring animosity for the countless atrocities committed against their people by pony-kind.
Information
(Unorganized information.)
The paws of a Hellhound can be used to create deadly unarmed weapons. Kage the Griffon was seen wielding such a weapon. Hellhounds are still intelligent, the majority are hostile to ponykind, but a few are willing to peacefully co-exist or at least leave Ponies alone.
Hellhounds also possess energy based weapons, exactly how they acquired them is still unknown.
Certain radio frequencies are also painful to Helhounds and will drive them into an unstoppable fury, until the source of the sound is destroyed.
Hellhounds are the wasteland equivalent of a Diamond Dog. They are smart, fast, and incredibly lethal. A Hellhound hunts best at night, and surprises it's target by tunneling to their location. Their heightened intelligence allows them to predict movements, set traps, and even handle firearms.
Hellhounds are very strong, and have been shown to be able to dismember limbs and tunnel through solid concrete with ease.
Hellhounds are weak to certain soundwaves. Like a dog with a dog whistle they will feel some level of pain and extreme aggressiveness and try to destroy the source of the high pitched sound. The Grand Pegasi Enclave have used ultra-sonic sound generators to heighten the aggressiveness of Hellhounds. One such experiment ended in the death of a Pegasi research team.
Notable Hellhounds
Albino Hellhound: A Hellhound possessed of a unique Albino coat. He was saved by Velvet Remedy and had one of his legs replaced with a cybernetic one.
Barkin Saw: An old Hellhound sniper living near Everfree. He is a near-deaf Hellhound which makes him immune to Enclave Transmitters. He joined Littlepip's assault on the S.P.P towers, riding on Spike's back alongside Calamity.
Fluffykins The Warclaw: An old female Hellhound. She was used as a test subject by The Pegasus Enclave to create the helmets used by the enclave to control Hellhounds. She was given tools to escape by Littlepip.
Project Horizons
A group of Diamond Dogs were sent to Hoofington to assist in digging tunnels. They helped build the vast tunnel system that spreads out across Hoofington. Even in post apocalyptic Equestria, these tunnels still stand, relatively intact.
The Hoofington Diamond Dogs did not become Hellhounds after the apocalypse. Rather their unique cybernetic bodies kept them as they were, possibly making them the last pure Diamond Dogs in Equestria.
These Diamond Dogs are descendants and survivors of pre-war Equestria. They need gem stones to power their cybernetics, otherwise they'd be unable to move. Rover a character from the show leads these Daimond Dogs, christened Sand Dogs due to the sandy area they inhabit.
They are willing to engage in trade with Ponies and do so in the town of Riverside.