Phione - the Sea Drifter Pokémon.
A Pokémon that lives in warm seas. It inflates the flotation sac on its head to drift and search for food.
As of now, Phione is in a sort of limbo. You can't get it through stories, the mart or park, and no one would dream of choosing it as a legendary Pokémon.
The main issue with it is the dispute over its legendary status.
Some claim it is, others claim it isn't. Even an official source, Pokemon.com's mailbag, when asked if it was a legendary Pokémon, the response was initially no, then quickly changed to yes, and then the answer was removed entirely. The official game guide to Heartgold and Soulsilver state that it isn't a legendary, as does Pokémon Ranger: Guardian Signs (stated in-game).
The only other Pokémon that is so disputed as a legendary is Rotom, with the main argument against that being that it can breed. Well, so can Phione. The difference with Phione is that it is obtained through breeding Manaphy, the only legendary Pokémon that can breed. But does this mean Phione is also legendary?
I say it doesn't. And several sources agree with me here.
It can't evolve into Manaphy through any means, it is for all intents and purposes a stand alone Pokémon.
But would it be unfair in the URPG setting? Is it in the same power level as the legendary Pokémon? No, it's not. It only has a base stat total of 480, 100 points lower than the legendary trios. It has a very limited movepool, it doesn't even learn Manaphy's signature move "Tail Glow" (which boosts special attack by three stages, which could be cause for concern). If anything, I don't see many people wanting to use it because of it's mediocre fighting potential.
Why should it be made available? There appears to be no good reason not to allow it. It's arguably not legendary, and it's weak to boot. The poor thing is so overlooked in general, it should be given its chance to shine.
As far as how to obtain it, it might be a stretch to make it available in the mart, but it wouldn't be entirely unreasonable (in the vein of Lapras, Pinsir and other single-stage Pokémon)
But my proposal is to make it at least available through stories, and perhaps the park as well.