It is said that long ago, back when Arceus (or in some versions Mew) was creating the pokemon, the first Sunkern, then among the greatest of the grass pokemon, petitioned Arceus to make his type stronger, for grass pokemon went hungry, being preyed upon often and having only rocks and soil to eat. Arceus disagreed, saying that he had balanced every type except for poison (which was to make up for in guile and subterfuge what they lacked in raw power) in his divine wisdom, and that grass had more advantages than weaknesses.
Sunkern, outraged by Arceus' refusal, gathered a large group of grass-type pokemon to steal away the Sun. The fire-types and Groudon joined forces with Arceus to protect the Sun, and Sunkern's point seemed proven by the overwhelming force of Grass's defeat. But while Virizion led the Army of Grass, Sunkern (now Sunflora) snuck past enemy lines and stole away a tiny and enormously bright piece of sunlight, while his comrades faced execution for treason against the gods.
Arceus, hurt, confused, and betrayed, put on a Meadow Plate and attempted to eat soil to determine how so many grass-types could turn on him, and at once he understood. The rebels were pardoned, and Arceus even allowed them to keep their piece of the Sun, feigning ignorance of Sunkern's theft, which they used to learn Solarbeam and create chlorophyll to eat sunlight.
But Arceus could not forgive their leader, and Sunkern and his descendants were cursed to forever be the weakest of pokemon, without even the speed to escape which has kept many a Magikarp alive today. Yet Arceus offered them one source of hope. Tiny pieces of the Sun were buried around the world, and any Sunkern who seizes one evolves into a Sunflora and escapes Arceus' curse.