A sudden transformation of Carbink, its pink, glimmering body is said to be the loveliest sight in the whole world.
– Pokémon X Pokédex
Diancie is an event legendary from gen 6. There was a movie featuring it. Despite being genderless, it has feminine qualities, and in the movie, was referred to as the Princess of the Carbinks.
Its Pokedex entry and interactions in the movie suggest that Diancie is closely related to Carbinks, perhaps is even a mutated Carbink. It has similar movesets to Carbink, and has the same base HP, Def, Sp Def, and Speed as Carbink (it has more Att and Sp Att). However, gameplay-wise there is no way to get a Diancie from Carbink. It would be cool though, if there was a very small chance to somehow obtain a Diancie from Carbink, perhaps via leveling up, or by breeding.
It’s the only event legendary Pokemon, and the only gen 6 Pokemon, to get a mega evolution.