In the Pokémon TV series, Ash sees Ho-Oh flying over Pallet Town in the first episode.
The moment occurs near the beginning of “Pokémon, I Choose You!”, after Ash wakes late and starts his journey with Pikachu. While traveling toward Viridian City, Ash looks up and sees a large, rainbow-colored Pokémon flying across the sky. Professor Oak later says that he does not recognize the Pokémon, leaving Ash with an unexplained first legendary encounter.
Ho-Oh is a Legendary Pokémon associated with rainbows, sacred towers, and the Johto region. It also appears prominently in the series opening sequence, which helped make the unidentified bird a long-running mystery for viewers.
Some viewers confuse this scene with Ash’s later encounter with Lugia in the Orange Islands or with the Legendary birds from Kanto. Those encounters happen in different stories. Ho-Oh is specifically the legendary Pokémon seen by Ash in the opening episode.