Legends say that Ho-Oh lived in the Tin Tower in ancient times. However, war broke out, and when the Brass Tower was burned, Ho-Oh created Raikou, Entei, and Suicune and then left. A new Tin Tower was created, but Ho-Oh never returned.
Ho-Oh completely colored and outlined in gold in its debut
Ash first saw Ho-Oh in Pokémon - I Choose You!. It appeared in a forest on his way to Viridian City during his first day as a Trainer. He was lying on the ground with his injured Pikachu and saw it fly over the rainbow that appeared in the sky after a storm. His Pokédex could not identify it and explained that many Pokémon were still unknown.
In Pokémon Emergency!, Ash tried to explain to Professor Oak what he had seen, though the image he used to point it out appeared to be of an Articuno rather than Ho-Oh. Either way, Professor Oak did not believe him and brushed it off as Ash being mistaken. This gold-colored version of Ho-Oh was an early concept of Ho-Oh as a Pokémon, as it, like all other Generation II Pokémon at that time, had not been fully developed yet.
The topic was brought up again in A Ghost of a Chance. While Ash was in Johto, he explained his encounter of Ho-Oh to Eusine, who also did not believe that Ash had seen this Legendary Pokémon. In For Ho-Oh the Bells Toll!, they heard the bells in the Tin Tower ringing, which would signal Ho-Oh's approach. At this point, Eusine was so upset by Ash's claim that he challenged him to a battle. In Pop Goes The Sneasel, it was seen using Sacred Fire in a flashback.
Ho-Oh's personality is unknown as all of its appearances involve it flying around but it appeared to have chosen Ash as one of the people for its visits as evidenced in A Ghost of a Chance.
Ho-Oh's appearance is that it is a very large Pokémon, and there is one shot in A Ghost of a Chance in the Tin Tower that shows three humans on their knees while Ho-Oh is perched up on a stand. Ho-Oh appears to be significantly larger than the humans.
Ho-Oh reappeared in I Choose You!, which is based in a continuity different from the main series.
Like in the main series, Ash and Pikachu witnessed Ho-Oh flying in the sky shortly after setting off on their journey. Unlike the main series, however, it dropped a Rainbow Wing as it passed by, with Ash picking it up. Keeping the wing with him, Ash set out to eventually meet and battle Ho-Oh. He eventually did, and it had a battle with Pikachu, the outcome of which was unknown.
In I Choose You!, it was seen flying over Ash and Pikachu after they dealt with a flock of Spearow and their leader. It dropped a Rainbow Wing in the process. Ho-Oh was then referenced to throughout Ash's journey to Mount Tensei where Ho-Oh will reside.
After Ash managed to place the Rainbow Wing on top of the rock, Ho-Oh appeared and Ash, his friends, Cross, Bonji, and the Pokémon in the area got to see Ho-Oh. Ash and Pikachu then battle Ho-Oh, but the battle was not shown.
In the Pocket Monsters the Movie: I Choose You! Remix manga
Ho-Oh appeared towards the end of ICYR1, where it flies over Ash and Pikachu, dropping a Rainbow Wing in the process.
This Ho-Oh was distributed to players who inserted a serial code that was found in the August 2017 issue of CoroCoro Comic, which was sold starting on July 15, 2017. The serial codes could be used to obtain Ho-Oh from July 15 to September 14, 2017. It was distributed to promote the 20th Pokémon movie.