Ash sees Ho-Oh in the sky at the end of Pokémon’s first anime episode.
After Pikachu defeats the attacking Spearow flock, Ash looks up and sees a large, rainbow-colored Pokémon flying above him. The creature is Ho-Oh, a Legendary Pokémon whose game data was not included in the original Pokédex available to viewers at that point.
Ho-Oh’s brief appearance became an early mystery in the anime. It was not identified by name in the first episode, and many viewers did not yet know which Pokémon they had seen. The character was later established as a major Legendary Pokémon associated with Pokémon Gold and Silver.
Lugia is another prominent Legendary Pokémon from the second generation, but it is not the Pokémon Ash sees in the first episode. Moltres is a first-generation Legendary Pokémon, while Rayquaza debuted much later in Generation III.