Bulbasaur is based on a frog. According to Ken Sugimori and Atsuko Nishida, the latter designed Bulbasaur working backward from Venusaur, meaning Bulbasaur was the last of its evolutionary family to have been designed and was also directly based on Venusaur. In particular, its build and ears resemble the Beelzebufo, a large prehistoric frog.
The bulb on its back resembles a lily or onion.
So it is based off a frog, created after Venusaur.
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