Plant Monsters are a limited time decoration in FarmVille. Plant Monsters were only available during the Halloween Event (2009) and can therefore no longer be purchased by farmers.
Plant Monsters were re-released for a limited time for the Halloween Event (2010).
They are most likely a parody of Audrey II from Little Shop of Horrors.
Plant Monsters also appear as random items when you redeem a Zynga Card from GameStop when the game is offering you about the Zynga Cards.