Ho-Oh is the Pokémon shown flying over Ash at the end of the first anime episode.
In “Pokémon—I Choose You!”, Ash and Pikachu see a large, rainbow-colored Pokémon flying through the sky after the storm and their encounter with wild Spearow. Ash does not know its identity at the time because Ho-Oh had not yet been introduced in the video games when the episode first aired.
Ho-Oh later became a Legendary Pokémon associated with the Johto region and the Pokémon Gold, Silver, and Crystal games. It is a Fire- and Flying-type Pokémon whose design draws on the phoenix and other legendary bird traditions.
The scene is easy to confuse with Lugia, another famous Legendary Pokémon from Johto. Lugia is associated with the sea and later received a major role in Pokémon the Movie 2000. Ho-Oh, by contrast, is the mysterious rainbow Pokémon glimpsed by Ash in the anime's opening episode.