You can try searching on educational websites like Crayola.com. They often have a variety of free printable colouring pages, and there's a good chance they might have ones for the Three Little Pigs story.
The big bad wolf is an interesting part to colour. You could make his fur grey or black and give his eyes a menacing yellow or red. The trees and the background scenery around the pigs' houses also offer a great opportunity to be creative with colours. For example, you can make the trees green with different coloured leaves, or if it's a spooky version, you can use purple or orange for the trees. The sky can be blue on a sunny day or grey if you want to show a more threatening atmosphere.
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.
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.
I recommend the following Pokémon novels to you. They all have legendary Pokémon!
1. "Elf: The Opening Ambush Hunter": The protagonist's first Pokémon was a small fire dragon.
2. [Pokémon: Lightning Bird at the beginning of the game: The legendary Pokémon Lightning Bird was drawn at the beginning of the game.]
3. "Musashi of the Elves": Musashi and his Abbot are also one of the classic characters in elf novels.
4. [Ten Artifacts of Pokémon Evolution]: There are divine beasts like the Three Pillar King and the Hell Dragon.
I hope you like my recommendation. Muah ~
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.
Team Rocket would be the most obvious ones. They're always after rare and powerful Pokémon or Pokémon - related items, so the legendary egg would be a huge target for them.
Often, Ash has to complete a quest. In a fanfic with Ho - Oh, Ash might have to find three special feathers scattered across the region. These feathers are said to be the key to summoning Ho - Oh. As he embarks on this journey, he faces many challenges and overcomes them with his existing Pokemon team. Once he collects all the feathers, Ho - Oh appears and, impressed by Ash's determination, decides to fight alongside him. Another way is through a prophetic vision. Ash might have a dream that tells him how to find and bond with a Legendary like Entei, and he follows this vision to make it happen.