We've had bi/quadpedal switches before that, colour schemes too. If anything I think Gen 4 was the tipping point. Mainly that the themes/dual types were more in addition to what they were and more obvious about the theme. Charizard was a fire breathing lizard/dragon all the way through, venasuar and bayleaf had plants that grew up with them,ect. Even blaziken's fighting type and proportions can be said to come from his chicken/roster side.
Infernape had Chinese literature from a plain fire monkey, empoleon's theme was there with emperor penguins less so with the steel type. Torterra had world turtles,ect from just a grass turtle and more of a focus on it's head leaf than it's shell and the ground type also kina just showed up he aslo bulked up more than some might have guessed though grotle was a fine indicator of size.
But pre gen 4 still had outliers, blastoise had mythology in warturtle that disappears as fast as he grows cannnons out of nowhere (well not quite nowhere there's still most likely a reason but it dosen't seem to be turtle related, p) and sceptile's plants are alot less consistent than bayleaf and bulbasaur. (I think cannons and arm leaf blades and tree tails are freaking awesome but still not very consistent) And typlosion and swampert have the bipedal/quad thing. (Plus fire splitting then regoining, and colour scheme shenanigans respectively)
Gen 5 on was even more obvious on the added themes though, but I think pignite was a better indicator of wht it would evolve into than monferno, and Oshawott at least had the schalop in hindsight so the theme was always there. (colour scheme and proportions didn't of course.) Just though I'd get that out there (been thinking alot about previous starter themes with the discussion lately, really should bookmark thier bublpedia page) Anyway, I'd just be careful with going that far back with what people thought.