Ho-Oh is the Pokémon first seen flying in the opening episode of the Pokémon anime. Ash spots the mysterious rainbow-colored bird in the sky while traveling with Pikachu after their escape from the Spearow flock.
The appearance was notable because Ho-Oh was not included in the original Pokédex data available in Pokémon Red and Green. Viewers therefore saw a legendary Pokémon before the series had explained its identity or origin. The moment helped create curiosity around the wider Pokémon world.
Ho-Oh later became closely associated with Johto and Pokémon Gold and Silver, where it serves as a major legendary Pokémon. It should not be confused with Lugia, the prominent legendary Pokémon from the second animated film and the cover of Pokémon Silver. Moltres is another legendary flying Pokémon, but it is part of the original Kanto trio.