What If Ash was in Pokemon Horizons?

The Ancient Poké Ball!

A barrier suddenly appears above Liko just before Amethio could snatch her. Liko looks to Ash and gives him and reassuring smile, Ash nods and turns his attention to his old foes.

"So twerp, you gonna let your friend come with us?" James taunted.

"Or will you finally hand us that Pikachu!" Jessie said.

"Be careful, you can only pick one!" Meowth recited smugly.

"Wobbuffet!"

"Well I choose neither!" Ash declared as he sent out his Quaxly.

Quaxly strikes the net to free Roy and Fuecoco, causing them to land safely on the sand. Pikachu then strikes the hot-air balloon and sends Team Rocket flying.

"WE'RE BLASTING OFF AGAIN!"

"WOBBUFFET!"

The barrier around Liko comes down as her pendant begins to glow alongside Roy's ancient Poke Balls. The three trainers gather together as Zirc and Onia's Skarmorys use Gust to encase them in wind, blocking Friede from helping out. 

Amethio's Corviknight lands as its trainer recalls it before sending out Ceruledge against the trainers. "Looks like he's not giving up." Ash observed, "We're not getting out of this without a battle."

"But I don't think Roy and I are strong enough to beat him." Liko seemed to hesitate before smiling, "But I'm gonna give it a shot!"

Quaxly looks up at Ash with a determined look, Ash nods and turns to Pikachu. "Hang in there buddy, if things go wrong, I'm counting on you."

The three Paldean starters face off against Ceruledge. Quaxly strikes Ceruledge in the face with Water Gun for super effective damage. Giving Sprigatito and Fuecoco the chance to follow up with Leafage and Ember. Sprigatito and Fuecoco split off to different sides to attack Ceruledge, only for their attacks to be cut with Psycho Cut that strikes both Pokemon. Quaxly sneaks up and slaps Ceruledge with Pound for some damage.

"It'll be better for you all to stop this endless struggle and surrender." Amethio taunted emotionlessly.

"Not a chance." Ash responded.

The Paldean starters stare down Ceruledge as their abilities activate. Overgrow helps Sprigatito launch a powerful Leafage that Ceruledge blocks. Giving Fuecoco an opening to use an Ember that's boosted by Blaze. Quaxly follows up with a Water Gun boosted by Torrent that deals much more damage than before. Ceruledge then uses Phantom Force that pushes the Paldean starters to their limits.

Amethio smirks as Ceruledge prepares the final blow, though both of Liko and Roy's treasure glow brighter than ever before. Roy's Poke Ball opens up and a beam of light emits from it that splits apart the clouds above. A black-colored Rayquaza emerges from within the Poke Ball and looks down at the trainers.

Liko pulled out her Rotom Phone. "Rayquaza, the Sky High Pokémon. A Dragon and Flying type. It has lived for hundreds of millions of years in the ozone layer. When it flies at night, it resembles a shooting star."

"Whoa, that's a Rayquaza." Ash's eyes seemed glued onto the legendary Pokemon. "But, it should be green. I haven't seen a black one since-"

Friede gets past Zirc and Onia to rejoin the children just as the Rayquaza summons a Draco Meteor that strikes the entire beach. The Exploreres retreat as Roy runs up to the Pokemon above him.

"Rayquaza!" Roy shouted as he held up the Ancient Poke Ball, "I'm your trainer and I command you to go back inside your Poke Ball!"

The Black Rayquaza looks down at Roy, seeming interested in the young trainer before flying off, leaving the trainers stunned at what they just saw.

The trainers gather at Roy's house to gather their thoughts. The children seem especially excited that they encountered a legendary Pokemon. "Hey Ash, you look like you knew about Rayquaza." Roy asked, "Did you see a Rayquaza before?"

"Oh yeah!" Ash nodded, "A friend of mine encountered one and I saw one go against a Deoxys. But they're supposed to me green. But I did battle with a black Rayquaza when I was in the Kalos region."

Liko and Roy pester Ash for his stories before Roy's grandfather approaches them. "Y'know, I heard that a black Rayquaza was once owned by an ancient adventurer."

"Interesting," Friede mused, "But why would a Roy have an Ancient Poke Ball that has such a Pokemon inside."

"I have no clue." Roy's grandfather admitted, "I simply found it on the beach when I was a around Roy's age and gave it to him. I didn't think there was a Pokemon inside, I thought it was empty."

"That's so cool!" Roy beamed.

Liko revealed her pendant to the elder, "Do you know anything about this."

Roy's grandfather shook his head, "Sorry, not a clue."

Friede's Rotom Phone begins to ring, "You have a call from: Mollie"

"Looks like Mollie's done treating your Pokemon." Friede concluded, "Let's go by the Brave Olivine and get 'em."

The trainers head onto the airship and wait outside the infirmary. Soon Sprigatito emerges and jumps into Liko's arms, then Ash's Quaxly runs out and hops onto Ash's head. Fuecoco emerges from the room and looks at Roy before spitting out a Poke Ball.

"Wait, do you want me to catch you?" Roy asked.

Fuecoco lets out an eager cry as Roy holds out the Poke Ball. Fuecoco taps the center-button and allows himself to be caught.

"Great job, Roy!" Liko praised.

"Well now that you have a Pokemon, you should join us!" Ash suggested.

Roy hesitated, "I want to, but I'm not sure my grandpa will let me."

"I will." A voice nearby said, the trainers turn around to see Roy's grandfather on the Brave Olivine. "I knew you'd want to adventure the world just as your parents had."

Roy smiled, "Thanks Grandpa!"

The RVT prepare the airship for their departure for Paldea as Roy returns the flag they lost thathe found. Roy watches as the Brave Olivine takes off, the people of his island waving him goodbye and wishing him safe travels.

Roy reunites with Ash and Liko. "I dunno when, but I'll find Rayquaza again!"

"I wanna find Rayquaza too!" Liko exclaimed.

"Me three!" Ash added.

The three trainers share and laugh as the Brave Olivine flies off into the sunset.

A/N: I know a lot of people like to believe most of the Pokemon movies aren't canon to the anime, but I like to think so (except M20 and beyond, and Detective Pikachu). The reason for my conclusion is that Lumiose Trainer Zac made a pretty informative video about which movies are canon that I would recommend.