The Bulbasaur line, as do all Gen1 starters, showcases the progress of evolution without getting too complicated. With no clear real-world inspiration, Venusaur is simply a miscellaneous dinosaur, a rowdy reptile. Venusaur LOOKS an awful lot like a toad with its long mouth, spaced eyes, and lack of a tail. However amphibians have no nails, and Venusaur has quite the pointy digits. And ears. Take THAT hypothetical Venusaur-is-actually-an-amphibian-debater.
Venusaur's greatly resembles the rafflesia, also known as the "corpse flower". It is large, red, and stinks of rotting meat. Venusaur probably reeks, and I suppose this explains it's poison typing. Stinky boy.
A very pleasing feature of Mega-Venusaur is how jungly its flower looks. Its just so neat. Granted, the body could have stood to change a little more, but its teeth are bigger so that's something.
Now you probably noticed that there are two Venusaur images above and are thinking I made a mistake. I can hardly blame you for not noticing Venusaur's gender variations. The difference lies in the yellow bulb on Venusaur's flower. The make variation is on the left, and the female on the right. This gender difference is lost in Mega-Evolution, both have no bud poking through the flower's center. An possible oversight. Or Venusaur briefly defaults to male when Mega-Evolving. Rapid gender-changes are far from uncommon in nature, bur that's a talk for another day (Maractus, I'm looking at you).
Aside from being a flower toad, many would hesitate to attach one specific creature to Venusaur's concept. Too bad, I already overthought it.
This good boy right here called a Kannemeyeria, an extinct herbivore from the Triassic period. If I was pressed to attach Venusaur's origins to a specific dino, it'd be this squat fellow, mostly for the leg structure. The actual Kannemeyeria was likely beefier than the one shown above, as most animals we only know from fossil records are inaccurately shrink-wrapped to their skeletons, but you can still tell that this is a very good boy right here.
Venusaur on its own has a very solid competitive standing as a bulky Chlorophyll sweeper, but really shines with its Mega. Gaining Thick Fat gets rid of two weakness and a boost to all stats with the exception of speed. Throughout the generations Venusaur has been the most consistently reliable starter competitively. Since the beginning it has been a bulky and good boy capable of running whatever the hell it wants too.
Now you're probably wondering why I have not gone into the ecology relating Venusaur. Its development in the wild, predators, prey, natural dilemmas that they would have to go through in order to survive as a species. Well, its hard to touch on these things due to the tricky nature of starter's and their place in the natural world of pokemon.
Plain and simple, the incredibly rare nature of starter's implies that they don't exist in the wild at all. They seem to have numbers that are so small they can only be kept in captivity and given to up-and-coming trainers to "start" them off. They are only ever seen outside of this in the Friend Safari. which is, again, a controlled environment.
Its entirely possible that Bulbasaur, Charmander, and Squirtle aren't even native to Kanto, and are instead outside species that were brought overseas.
As a closing note, ever wonder what Venusaur looks like with a closed flower? Wonder no longer my good friend.
Wonder no longer.