Originally Posted by goldwynaut
But bulbasaur is a frog, even if its name has saur in it, and it evolves into a huge fat toad.
How is Bulbasaur a frog?
The definition of amphibian is "a cold-blooded vertebrate animal of a class that comprises the frogs, toads, newts, salamanders, and caecilians. They are distinguished by having an aquatic gill-breathing larval stage
followed (typically) by a terrestrial lung-breathing adult stage."
The bold part is what characterises an amphibian. To be an amphibian it needs to have a stage of its life in water, which it clearly doesn't.
Venusaur is more like a Dicynodont than a toad. Even then, it's hard enough to try to classify one Pokemon into some kind of earth species. It's almost impossible when taking into account a whole evolutionary line, because Pokemon don't follow our rules of nature. You have eggs evolving into trees, fish into dragons, sheep into kangaroos, fish into octopuses, clams into dolphins...
Mammals, amphibians, reptiles etc. don't even exist in the Pokemon world. But all of the Grass starters before Chespin were based on earth reptiles.