Cherrim

Cherrim, art by Ken Sugimori. Its Sunshine form clearly had too much sugar.

Cherrim is an official Grass-type Pokémon introduced in Generation IV. It is #421 in the National PokéDex.

It is the evolved form of Cherubi and is part of the evolutionary line eventually leading into Cherisol.

Not even Game Freak can explain how the hell does it change from its Overcast form to its Sunshine form. Shenanigans and stunt doubles may or may not be involved.

For more information on Cherrim, see its Bulbapedia entry.

Evolutionary Data:
Cherubi → Level 25 → Cherrim → Level up while knowing Sol Force → Cherisol