Ho-Oh was the first Pokémon shown in the original anime that was not in the original Pokédex.
In “Pokémon, I Choose You!”, Ash sees Ho-Oh flying across the sky after Pikachu helps him and Ash escape a flock of Spearow. The moment is brief and mysterious, and neither Ash nor the viewers are given its name in the episode itself.
Ho-Oh was not part of the 151 Pokémon introduced in the first-generation Game Boy games. It debuted as a Generation II Pokémon in Pokémon Gold and Silver, where it is associated with the Tin Tower in Ecruteak City. Its early anime appearance therefore acted as a glimpse of a Pokémon from the next generation before those games were released.
This fact is often confused with Mew, which was hidden in the original games and belongs to the first generation. Lugia is also a Generation II Legendary Pokémon, but its major anime role came later in Pokémon the Movie 2000. The episode's mysterious flying Pokémon is specifically Ho-Oh.