We usually see evolution as the process of things learning to stand. Fish, which have a hell of a time walking, become amphibians and reptiles, which scrape across the ground. These evolve into mammals, which can dance and wear pants and everything.
In Red/Blue, the sprite for Ivysaur definitely reflects this evolutionary idea, as Ivysaur is standing up, and looks like the kind of Pokemon that would tackle you if you changed his Eagles song on the bar jukebox. Stocky and oddly human, his pose is kind of mean. He looks like one of Doctor Moreau’s hybrids. Bulbasaur, on the other hand, seems like he’s just eager to sit in your lap. Ivysaur is looking for any excuse to wrestle something. Bulbasaur has a hard time getting onto the couch.
And I love that, which makes me sad that they plopped Ivysaur back down onto four legs for every subsequent sprite. On two legs, he’s a badass. On four legs, he’s just Bulbasaur with more leaves. They did this starting with Yellow, and it’s the lamest course correction in the series. Sure, there are a lot of Pokemon sprites in Red/Blue that needed fixing, as they looked less like Pokemon and more like signals that your Game Boy was haunted. But Ivysaur was the one mistake that was cooler as a mistake.