When people say Venusaur is a "Dinosaur" they aren't referring to dinosaur, there is no dinosaur with external ears, fangs (they had sharp teeth, but no proper fangs), or true elephantine legs, DIICTODON THOUGH

Diictodon was part of the bridge between mammals and reptiles, is a close relative to the IRL Bulbasaurus, has fangs, potential external ears, stumpy legs. A young diictodon IRL would look a lot like bulbasaur given the shorter body