@QuadFactor It's one of those oopsies that plague the early years of a franchise (think Mario's overalls for example). Basically:

* Venusaur's main FRLG sprite is directly based on its 'Red and Blue' artwork, which shows it with five petals. Its in-game sprite also has five petals in JP Blue/RB.
* In 'Red and Green', Venusaur has six petals, both in-game and on its main artwork, which was used on the games' boxart (which is directly referenced in the FRLG credits).
* The RG artwork was used as a basis for its animated series' model sheet, so Venusaur has six petals (also the case in 'Pokémon Yellow', whose sprites are largely based off of anime renders).


* In 'Gold'/'Crystal', Venusaur has five petals, perhaps because it used the RB art as a basis.
* In 'Silver', Venusaur has... six petals. That sprite was made later, and most likely by a different artist (see the dithering) so they likely used the RG art as a reference.


* Gen 3 mainline stuck with five petals, but the in-game design was consolidated into six petals by the time Gen 4 rolled around.

It's weird how the venusaur sprite with six petals is the unused one. I wonder if the five-petal design was going to be a region-based thing like the different pattern on arbok, or maybe they simply forgot; it's anyone's guess.