Articuno, Zapdos, and Moltres are quite common. Their elemental powers and the mystery surrounding them make for great fanfiction material. Ash might encounter them during his travels and have to either help them or face off against them in some way.
One of the Legendary Pokemon could be Giratina. Despite its somewhat dark and otherworldly nature, Giratina might be intrigued by Ash's light - heartedness and his ability to make friends with all kinds of Pokemon. Dialga and Palkia could also be part of the harem. Dialga, with its control over time, might see Ash as an important factor in the future of the Pokemon world. Palkia, in charge of space, could be interested in how Ash navigates different regions and spaces in his journey. And don't forget about Ho - Oh. Ho - Oh is a symbol of hope and might see Ash as a bringer of hope in the Pokemon world, thus joining his harem.
Arceus is quite popular. Given that it's considered the creator of the Pokemon universe in the official lore, it often has a significant role in fanfictions. Mewtwo is also very common. Its powerful psychic abilities and complex backstory make it a favorite for fanfic writers to explore. Another popular one is Lugia. With its majestic appearance and its connection to the sea, it's often a central figure in many stories.
In 'The Lord of the Rings', Frodo, Sam and Gandalf can be considered a sort of trio. Frodo is the ring - bearer, Sam his loyal friend who stays by his side throughout the journey, and Gandalf is the wise wizard guiding and protecting them in many ways. Their combined efforts are crucial in the fight against Sauron.
Mewtwo is also very popular. Given its origin as a genetically - engineered Pokemon, a human could interact with it in many ways. Maybe a human scientist wants to study Mewtwo further, or a young trainer tries to befriend Mewtwo and help it come to terms with its existence and powers.
There are also fanfictions where Ash catches Rayquaza. This could be set during a time when the sky is in chaos due to some dark energy. Rayquaza is drawn to Ash because of his pure heart and determination. Ash, with his usual courage, teams up with his pokemon to assist Rayquaza in restoring balance. As a result, Rayquaza allows Ash to catch it.
One example is the Golden Trio in the 'Harry Potter' series, consisting of Hermione Granger, Ron Weasley, and Harry Potter. Hermione is highly intelligent and a strong female character. Another is the three witches in Shakespeare's 'Macbeth'. They play a crucial role in the prophecy and the downfall of Macbeth. And in 'Charlie's Angels', the three female agents are a well - known female trio, showing strength and intelligence in their detective work.
In 'The Three Musketeers', Athos, Porthos and Aramis form an iconic trio. They are brave, loyal and full of chivalry. They fight together against their enemies, always having each other's backs in the face of danger and intrigue in the French court.
In Ash's adventures in legendary Pokemon fanfics, he often has extraordinary encounters. For example, he might form unique bonds with legendary Pokemon that are not part of the main canon. These fanfics can take his story in new and exciting directions, like having him team up with a Mewtwo in a way that defies the normal Pokemon - trainer relationship. It's all about exploring the untapped potential of his character in the context of the amazing world of legendary Pokemon.
The appeal lies in seeing Ash in new and exciting situations. With legendaries involved, his journey becomes more epic. It's like taking the underdog story of Ash in the anime and supercharging it.