Ghosts were a unique type of abomination that appeared during the first month of the fallout. They all wear the same yellow Mark VI Omni-Environmental Suits which help sustain them.
When the megaspells and balefire began falling, numerous Foals in Canterlot were placed inside Mark VI environmental hazard suits. The suits had the most up to date medical and logic spells built into them, all in an attempt to save the ponies inside. The suits proved unable to resist the extremely high levels of radiation or the Pink Cloud which both penetrated the suits.
The Foals wearing the suits were subjected to a form of merging process, where the souls of the wearer were bound to the suit by a Zebra talisman built into the suit. This prevented the ponies wearing them from dying, but also meant they were bound to their suits.
About a month after the Megaspell Fallout, the Ghost Herd emerged from Canterlot, made up of Foals obsessed with finding their parents. Many had gone mad due to their transformation, whilst others followed in confusion or fear of being alone. The Ghost Herd passed through Route 52, heading south. They killed any living creature they came across, their suits continued to repair and regenerate their bodies.
Eventually the Ghosts vanished completely into a mere myth, many of them had been killed by ponies defending themselves, with only the Ghouls from around 200 years ago, knowing anything about them. The Last Ghost to awaken in Canterlot was Puppysmiles in modern day, post apocalyptic Equestria. Like the rest of the Ghosts, Puppy was obsessed with finding her mother and traveled down Route 52, making many friends and putting an end to numerous conflicts and evils (Though often unintentionally.)
Three other Ghosts survived the last 200 years, with two of them being killed by The Herd, an army of Tribals & Raiders, that was moving North down Route 52. The last surviving Ghost was nicknamed Sidekick by Puppysmiles who followed Puppy to her mother's grave and was controlled by Creepy Voice and orderd to attack Henrietta. Puppysmiles and Sidekick both died, meaning the Ghosts are most likely extinct.
In other stories
Ghosts appear outside the Casino Royale. One is notably continuing the tasks it had before the war, acting as a janitor and cleaning up various bits of rubbish as well as the corpses of fallen adventurers. They're considered ideal prey by God, who deems them soulless.
The New Pegas versions can be killed by head shots from sufficiently powerful weapons, although not by any other means. While the hazmat suits they wear are similar to those of the Canterlot Herd, they are clearly a different model, more focused on preservation of the wearer and seemingly without built-in A.I. support. Being capable of reconstituting themselves even from ash piles, it is likely that they are protected to some extent by Phoenix-based magic.
A Ghost is either a Ghoul inside their suit or in the case of Puppysmiles, a mass of pink ooze that fills the suit with Gas to take on the appearance of the Foal inside. Ghosts can suffer Gunshots and other horrible injuries that would normally kill a regular Pony. The only way to fully kill one, is to destroy the talisman binding the soul to the Environmental Suit.
Creepy Voice was able to use Puppy's gaseous body to perform impressive attacks, creating gassy tendrils that it used to attack SolOS's robots and later Puppy's friends who had come to help her, at the end of her journey. Ghosts have shown impressive feats of strength, as shown when they literally dismembered raiders from the Herd. Puppysmiles was also remarkably strong, being able to use a rock to smash The Pinkbot at The Carnival and Sentinels in Tunnel town.
Creatures like Paradores are terrified of Ghosts, as shown when Puppysmiles tried to hold and hug one.
- The Ghosts are based off of the Ghost People that populate the Sierra Madre in Fallout: New Vegas
- The yellow suits they were are similar in appearance, to the Radiation suits from the Fallout series