Vileplume...OK, there's not much to say except, well, it evolves from the extremely common Oddish. How can that be good? Well, it learns Aromatherapy, so that's something.
The plant that Vileplume is based off of, Rafflesia arnoldii, has the biggest flower in the world. Its massive petals can stretch up to a meter (roughly a yard) across and a single bloom can weigh up to 24 pounds. It is found only in the rain forests of Sumatra and Borneo and is considered endangered. Several other Rafflesia species also went into Vileplume's composition.
Guy shown for scale. Lots of other cool pics here.
True to the rotten stench of Gloom, the evolution between Oddish and Vileplume, the rafflesia is also affectionately called 'corpse flower.' It reeks like a dead body so that insects will come pollinate it. Given that rafflesia can be male or female and will only bloom for a short time, successful pollination is a little tricky.
It gets weirder: Rafflesia lack many of the traits commonly associated with plants such as roots, stems, or leaves. It instead gets its nutrients by parasitizing a Tetrastigma vine. The only thing plantlike about this flower is, well, the flower. Otherwise, it resembles a fungus.
No, this plant is not a triffid, but it's still pretty damn weird.