Venusaur did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Venusaur are a dual-typed Grass and Poison Pokémon species whom are the final evolutions of the starter pokemon Bulbasaur, it evolves from Ivysaur at level 32. It is the only Pokémon that can learn the move Frenzy Plant until Pokémon Diamond and Pearl where Venusaur, Meganium, Sceptile, and Torterra could learn it as well. They are the third Pokémon in the PokéDex.
It can Mega Evolve into Mega Venusaur. The required Mega Stone for this Pokémon is called Venusaurite.
Venusaur have large Pink flowers on their backs and they have short chubby legs.