Ho-Oh was the first legendary Pokémon Ash saw in the Pokémon anime.
Ash sees the mysterious bird flying over a rainbow near the end of the first episode, “Pokémon, I Choose You!” At that point, Ho-Oh was not identified by name in the episode and was not included in the Pokédex data available to viewers.
The sighting became an early piece of anime foreshadowing. Ho-Oh was not one of the original Pokémon revealed in the first games at the time of the anime's debut, so its appearance created speculation among fans about the creature's identity.
Ho-Oh later became a major legendary Pokémon associated with Pokémon Gold and Silver. It is a Fire- and Flying-type Pokémon and serves as the mascot of Pokémon Gold, while Lugia serves as the mascot of Pokémon Silver.