6th Gen Carbink and Diancie

[Howmander]

Technically Manaphi is a mutation of Phione. I'm not exactly sure were this was mentioned, though.

From what I heard actually, Manaphy can breed like a normal genderless pokemon, but they can only breed Manaphy in the cold waters of the Rangers regions, whereas in the trainer regions the water is too warm and they breed Phione instead.

 

[PokeXavier]

They were probably first the same pokemon and arceus split them apart

 

[adameastment]

As others have said Diancie is a mutation.

I don't think we will ever be able to get Diancie within game, as if it is a mutation then it is born from a Carbink. I don't think that Game Freak will introduce a mechanic that will randomly create a Diancie when you breed hundreds of Carbink eggs.