Haven't been able to rid this more out there theory from my head, so...
What if Ogerpon is a bespoke, one-of-a-kind Pokemon created by Terapagos (possibly in tandem with "Dokutaro") as wish/request fulfillment?
As far as I've heard, Ogerpon's trainer was merely not from Kitakami and clearly looked like an outsider. Saying that trainer is from Paldea is not a bad guess considering all the Tera crystals they dumped in the lake. It's implied that even auto-Terastallization of wild Pokemon outside the Great Crater of Paldea may have only started occurring after Prof. Sada/Turo started their research (although it's also said that Tera Raid dens are merely created from the Great Crater's power "leaking over time"), so it's very possible that Ogerpon's trainer went into the Great Crater themselves to get Tera crystals since they're from before the Professor's time.
And while they're getting Tera crystals, if they bump into Terapagos (in not-lazy mode)...why not try to get one more boon while they're at it?
There's no word of Ogerpon's kind using to exist in Paldea, which is often a sign that kind is very reclusive or very rare (or at least the real-life equivalent is - note recently scientifically discovered animals like the
Laotian Rock Rat being mentioned by the locals as "bush meat" for ages, implying the common, non-reclusive animals get pegged by the locals far earlier than "undiscovered until just now"). What about so rare that
the only Ogerpon in this Pokeverse is the one who moved to Kitakami?
Someone with a powerful enough imagination (think Takato from Digimon Tamers coming up with Guilmon) can easily come up with a cute, starry-eyed, horned, orange-inspired "fakemon", request one in front of Terapagos, and watch that "fakemon" become real.
It's implied that you have good reason to go back to Kitakami in the Indigo Disk and talk to the newly appeared zombies (and maybe hit up Perrin one more time); maybe one reason is to investigate more of Terapagos's handicraft...and that handicraft isn't just Toxic Chains.