Petilil was officially revealed on the October issue of Coro Coro on September 11, 2010.
Petilil bears similarities to Oddish. They both can evolve by a Sun Stone into their fully evolved forms, Oddish can evolve into Bellossom while Petilil can evolve into Lilligant. Their final evolvtions seems to have a flower on top.
Unlike Petilil's counterpart for Pokémon Black, Cottonee, Cottonee can be both male and female while Petilil is always female and can't be male. Both of them can evolve by Sun Stone and in their evolved forms, they can learn new moves at lv 10,28 and 46.
In both games, the Player can give a name to a Petilil. In Pokémon Black, the player needs to breed Lillil, the Player's Lilligant (Dye's trade, then evolve Lillil), then name the Petilil. In Pokémon White, it works like any other catch of a wild Pokémon.