Before the War
Phoenixes were birds with red and gold plumage that populated Equestria and likely other parts of the world. They were carnivorous creatures that preyed on small animals like mice. They were also known to be kept as pets; Princess Celestia herself owned one.
After the War
As a result of incineration due to rejuvenating themselves and severe exposure to radiation most, if not all of phoenixes became balefire phoenixes. Many of these birds originate from the now-destroyed wildlife sanctuary 'Manehattan Gardens', which was struck by the balefire bomb that hit the city.
They were left largely unharmed by the transformation, however they have changed their coloration to green and gold, and now breathe green, radioactive fire. Like pre-war phoenixes, they possess the ability to rejuvenate themselves from their ashes if incinerated, but otherwise are as mortal and vulnerable as any critter.
Apperances in Other Stories
Titular character Ditzy Doo and her companion May, encounter a Balefire Phoenix in Manehattan. The phoenix was notably large due to soaking up radiation and promptly attacked the two ghouls. Ditzy and May managed to kill/incapacitate the phoenix by causing a building to collapse on it.
Balefire Phoenixes are covered in green and gold plumage. They take on a green glow when they absorb excess amounts of radiation.
Balefire phoenixes are carnivorous and have predating habits of small prey. Like most creatures which were transformed from radiation, balefire phoenixes are immune to the threats it normally poses. Instead, radiation has a healing factor on them, and additionally, balefire phoenixes are able to store excess amounts of it inside their bodies for several days, wearing a large, green glow around themselves, and having their fire breathing greatly enhanced in power and range.
Pyrelight - She is a balefire phoenix that was following Littlepip's group. She eventually joined the group as Velvet Remedy's pet and companion. Pyrelight proved to be a helpful ally on many occasions and showed great intelligence a number of times.
Philomena - She was Princess Celestia's pet before and during the war and presumably perished due to the pink cloud in Canterlot or survived as a balefire phoenix. Pyrelight bears a remarkable resemblance to Philomena, hinting at some kind of family connection between the two.