What If Ash was in Pokemon Horizons?

Hatenna and the Otherworldly

Ash, Liko, Roy, and Dot trek through the forest, "I'm telling you guys, The Otherworldly has gotta be around here somewhere." Roy insisted.

Suddenly the trainers hear footsteps coming from the direction they're heading in. Coming face to face with three trainers that look similar to Liko, Roy and Dot.

"You guys okay?" Liko rushed over to help them.

"Yeah, just be careful heading over there." the girl looking like Liko said. "I'm Niko, that's Rob and Dash. We're part of the Tera Training course at Naranja Academy. We go lost and ended up at this spooky-looking tree, the strap of my bag broke and I used a vine to fix it and we found a scary and strong Pokemon that attacked us. It was too strong for us to battle, so we had to run away."

Roy beamed, "That's gotta be The Otherworldly! Can you tell us where you found it? We wanna write a report about it."

The three trainers tense up and run away, "No thank you! You've got the World Monarch!"

The trainers venture deeper into the forest, noticing a spread of fog rolling in. Soon a giant Pokemon stands before them hidden in shadows. "Okay, this has gott be The Otherworldly."

Liko scans it with her Rotom Phone, "Annihilape, the Rage Monkey Pokémon and the evolved form of Primape. A Fighting and Ghost type. When its anger rose beyond a critical point, this Pokémon gained power unfettered by the limits of its physical body."

"Wait, Primape evolves into that?!" Ash exclaimed, thnking of his own Primape.

Annihilape strikes at the trainers, forcing them to send out their Pokemon to help. Dot's Quaxly and Floragato hold it back while Fuecoco and Pikachu strike with Flame Charge and Iron Tail, knocking it down. The trainers make a run for it, stopping to catch their breath at a nearby river.

They spot a group of Mankey sitting next to a tree that dons a scary face in its bark. One Manky holds a snapped vine. "That must be the tree they were talking about, and that must've been the vine Niko used."

"Hey, you're right." Dot observed. "That this tree must be important to the Mankey somehow."

"Plus you guys look like Niko and the others." Ash added, "Maybe they think you came back to do more harm."

The Mankey spot them and begin throwing rocks in their direction. Several Primape charge at them as well, forcing the trainers to defend themselves with their Pokemon. One Mankey throws a rock that almost hits Terapagos, but Hatenna uses her Psychic powers to deflect it. Annihilape appears again, and the Mankey and Primeape move aside to make way for it. 

"Hey, the Mankey and Primape stopped when Annihilape came back." Dot theorized, "Myabe it's their leader." 

Annihilape attacks with a flurry of Shadow Balls, and the trainers' Pokémon fight back, but their attacks don't do a ton of damage. The Pokemon simultaneously attack with Magical Leaf, Flamethrower, Water Gun and Thunderbolt. Which only makes Annihilape angrier.

While Annilhape battles against the partner Pokemon of each trainer, Hatenna spots some Mankey staring at a tree on the opposite shore. Hatenna goes up to the trainers and points to the tree, and they spot many berries growing on the tree. "Y'know I think that may be how the Pokemon here get their food." Roy said.

"And the vine Niko used must've been how they got back and fourth." Liko added.

The trainers call their Pokemon back and work together to build a bridge out of straw and logs. Hatenna attaches the bridge to both sides of the shore as a finishing touch. The wild Pokemon excitedly cross the bridge and dance around the berry tree in celebration, which causes a loose boulder to fall down from the cliff above. Everyone rushes to get out of the way, by the time the boulder reaches the water and sinks, the berries have all been smashed. The wild Pokemon fly into a fit of rage, causing two Primape to evolve into Annihilape. The Wild Pokemon begin to fight each other, the three Annihilape knock out the wild Mankey and Primape and begin fighting each other. The trainers fall back, though Liko notices Hatenna approaching the Annihilape.

"What's she doing?!" Liko asked in worry, "She's gonna get hurt!"

"She's gonna try to stop it." Ash said, "I believe in her."

Hatenna reads the heightened rageful emotions and quickly gets overwhelmed and begisn to glow. "She's evolving!" Liko exclaimed in excitement, her and Ash watching proudly.

Hatenna remerges with a larger hat and visible eyes, which Dot scans from her Rotom Phone. "Hattrem, the Serene Pokémon and the evolved form of Hatenna. A Psychic type. Using the braids on its head, it pummels foes to get them to quiet down."

"Pummels foes-" Dot asked before hearing a loud crash, seeing Hattrem swing its braids bathed in a dark glow to knock out all three Annihilape. "Oh yeah, that's Brutal Swing!" Ash beamed.

Hattrem heals the Annihilape right afterwards, much to Dot's suprise. The trainers pick some new berries for the Pokemon and Dot snaps a photo of the wild Pokemon eating. Hattrem attempts to climb into Liko's hoodie and is dismayed to see she no longer fits inside. "Sorry Hattrem, looks like you have to go back in the Poke Ball." Liko recalled Hattrem as she made her way back to Naranja Academy alongside her friends.