"Ah, Ash!" Professor Elm said. "Professor Oak told me you were on the way… did you speak to him recently?"
"Yeah!" Ash agreed. "He knows I'm coming through New Bark before heading on to the Silver Conference!"
"Well, he asked me to speak to you," Elm clarified. "Apparently there's a Pokémon he's ready to transfer to you? And he'd like a video call. Then I've got something to ask you myself, but that can wait."
"Got it," Ash confirmed. "Let's go, Pikachu!"
"Pika!" Pikachu agreed.
"Ash, it's good to see you again," Professor Oak said. "Now, this is an unusual situation, so I wanted to make sure you understood it."
"I guess that's better than not understanding it," Ash replied.
"Of course," Oak chuckled. "The first thing I need to make sure you understand, Ash, is how rare it is that a trainer has a Pokémon like this for a Pokémon League. It's not unknown, but it's rare enough that you're going to need to be careful. You're already a target for Team Rocket, but this could make it worse."
"I've got it, Professor!" Ash nodded firmly. "I won't let Team Rocket take any Pokémon while I'm around, especially not Pokémon I'm responsible for!"
Pikachu nodded firmly. "Pika Pika-chuu!"
"That's good," Oak said. "Now, the second thing is – if you have a very strong Pokémon on your team, you have to avoid the temptation to just have them do everything. That's not fair on anyone on your team. You need to help them grow and get stronger, and you need to do it without neglecting the rest of your team. I hope you can do that?"
"I'll do my best, Professor!" Ash confirmed.
"That's good, Ash," Oak reiterated. "I know you're a trainer with a good heart, I just need to make sure. I'd say the same if I was sending a Pokémon like this to Gary. And, finally… you realize that you can't keep a Pokémon like this forever? You'll need to let them go again eventually."
"Pika, Pikapi!" Pikachu giggled. "Pikachu-chu!"
Ash blushed. "Yeah, I do let my Pokémon go if they want to… or sometimes even if they don't, if I'm sure it's what's best for them. But Charizard's doing really well now so it must be working!"
"That's all my worries, Ash," Oak said, then pressed a button.
The Pokémon Transfer System flashed into life, and a Pokéball appeared on the machine next to Professor Elm's computer.
Ash took it, and as soon as he'd picked it up it opened with a flash of light.
A white-and-pink Pokémon materialized, and Professor Elm gasped.
"That's the Pokémon Professor Oak wanted to send you?" he asked. "Ash – that's a Latias. That's a Legendary Pokémon!"
Latias giggled.
"Yeah, Professor Oak and I talked about it before!" Ash explained. "He gave her a check up, and she liked the sound of coming with me, and I guess this was once he was sure it was okay! There's rules about it, maybe… but, I'm going to be teaching her how to battle properly. Uh…"
He checked his Pokédex. "Professor Oak says she only knows Safeguard?"
The floating Dragon-type looked a bit embarrassed.
"Well… that sounds like you've got a lot of work to do," Elm said. "It must be fascinating, though… ah, now, the thing I wanted to talk to you about was an egg…"
AN:
This is fundamentally predicated on the idea that the Altomare Latias could have joined Ash in the anime for at least the rest of Johto, reportedly something they were planning but scrapped. I decided to see if the "wait, who the heck is this Pokémon" thing could be handled in a sequence that could fit in the anime, whether or not the viewer had seen Pokémon Heroes.