Yuqsdug
After 15 mostly fun years in the Pokemon world Alder, a human turned Treecko then Grovyle, had finished the task wherefore he was summoned into this world. And since his task was finished, his reason for being in this world was gone. His form quickly vanished as his soul was sent to the Gate between Worlds.
His summoner convinced her higher-ups to keep Alder and they agreed. But as his soul descended back onto the Pokemon world, it was snatched up into the human world like a fish on a hook.
And now, Alder is in a new, unfamiliar world in a new form. He is more upset that it's the second time this happened. This form may be a bit uncomfortable, but at least he kept his memories this time!
Or: the Protagonist of Pokemon Mystery Dungeon Rescue Team gets sent back to the human world instead of staying. Where he gets reborn as Izuku Midoriya. The forcefulness of his "Reincarnation" ends up eventually creating some fun Treecko mutations.
(Crossposted on AO3)