Perhaps in the fanfiction, Ash catches Ho - Oh as part of a prophecy. There could be an ancient prophecy that says a trainer with a pure heart (which Ash is) will catch Ho - Oh during a time of great danger. So, when this danger arrives, Ash manages to catch Ho - Oh and they work together to save the Pokemon world from the impending threat.
The plot might involve some kind of special event or place. For example, Ash could be in a mysterious ancient forest where Ho - Oh is known to appear. He might have to complete a series of challenges set by the forest spirits to prove himself worthy of catching Ho - Oh. And once he catches it, he has to learn how to train and bond with such a powerful and unique Pokemon.
Imagine a scenario where Ash is in a different dimension or a special realm where the rules of Pokémon catching are a bit different. He meets Ho - Oh there, and instead of using a traditional Poké Ball, he has to use a special item or perform a unique ritual. This would make for a very interesting and unique fanfic as it deviates from the normal Pokémon - catching methods we are used to.
One possible fanfic could be that Ash is on a journey through a mysterious forest. He senses an otherworldly presence. Suddenly, Ho - Oh appears, its feathers glowing. Ash, with his unwavering determination, throws a Poké Ball. Against all odds, the Poké Ball shakes once, twice, and then clicks, signifying he has caught Ho - Oh. This would completely change the dynamic of his journey and his status as a Pokémon trainer.
The Pokemon world would be in awe. Ho - Oh is a symbol of hope and legend. Ash catching it could inspire more people to become trainers. It could also lead to new research on legendary Pokemon. Scientists might want to study how Ash managed to catch Ho - Oh and what kind of bond they have. This could lead to new discoveries about the nature of Pokemon and their relationships with humans.
A possible scenario in a fanfiction is that Ash is given a special mission by Ho - Oh which involves his starter. His starter might have to overcome some challenges that are related to Ho - Oh's powers or the lore surrounding it. For instance, if Ash's starter is a Charmander, it could learn to breathe a new type of fire inspired by Ho - Oh's sacred flames.
In fanfiction, the relationship between Ash, Ho - Oh, and his starter can be very creative. Ash's starter might be a rare or unique one that has a connection to Ho - Oh's legend. Ho - Oh could see Ash's pure heart through his relationship with his starter and choose to guide them on their journey. This could lead to Ash uncovering secrets about the Pokemon world that are related to Ho - Oh and the starters' origins.
One popular 'Ash Catches Ho - Oh' fanfiction might be the one that focuses on Ash's journey to prove himself worthy of catching Ho - Oh. It could involve him facing various challenges and training hard. Another could be a more emotional take, where Ash has a deep connection with Ho - Oh before attempting to catch it, like sharing dreams or premonitions.
The plot could involve Ash hearing rumors about Mewtwo's location. He sets out on a journey to find it. Once he does, he tries to communicate with Mewtwo. Mewtwo initially resists but Ash's pure intentions start to sway it.
A plot might be that Lugia is being hunted by an evil organization. Ash discovers this and decides to protect Lugia. He uses his Pokémon to fight off the bad guys. In the process, he forms a bond with Lugia. Eventually, Lugia realizes that it's safe with Ash and allows itself to be caught. This would show Ash's heroism and his love for Pokémon.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own (AO3). These platforms have a large collection of Pokémon - related fanfictions, and there's a good chance you'll find some 'Ash Catches Ho - Oh' ones there.
Ash could catch Latias in a place full of ancient ruins. Latias might be guarding some sort of Pokemon relic there. Ash shows respect for the place and Latias' role. This makes Latias interested in him. Eventually, Ash catches Latias and they go on to discover more about the relic's power together.