A phoenix is a magical bird that regenerates by bursting into flames and being born anew, as the first phoenix introduced in the series, Philomena, does at the end of A Bird in the Hoof.
Depiction in the series
A Bird in the Hoof
The first phoenix in the series is introduced in A Bird in the Hoof, though throughout the episode the characters are not aware that the bird is a phoenix. Princess Celestia introduces Fluttershy to her pet bird Philomena, who appears almost featherless and sickly. Throughout the episode Fluttershy attempts to nurse Philomena back to health, but Philomena keeps molting, coughing, keeling, and generally acting sickly. Eventually, Philomena runs away and soon bursts into flames and collects into a pile of ashes on the ground, much to Fluttershy's horror and other spectators' dismay. Princess Celestia, who arrived moments earlier, explains to Fluttershy that phoenixes occasionally renew themselves by bursting into flames and rising from the ashes left behind. Philomena pokes her head out from the ashes and rises into the air in brilliant red plumage and a transformed appearance.
Spike encounters more phoenixes in Dragon Quest, and eventually adopts one. In the episode, a group of adolescent dragons persuades Spike to participate in raiding a phoenix nest for eggs. Spike distracts the parent phoenixes by throwing a rock at them, prompting them to chase after him. At the same time, the other dragons invade the nest. Instead of finding unhatched eggs, they find a group of hatchlings, who manage to escape. The dragons give chase, but the parent phoenixes blind their pursuers with a flash of light and escape with their hatchlings. One unnoticed unhatched egg is left behind and discovered by Spike, who rescues the egg and later names the hatched phoenix Peewee.
The Elements of Harmony guidebook
A PHOENIX is a mystical bird that sheds its feathers and is reborn in an explosion of brilliant flames.