Bulbasaur, a Grass/Poison-type Pokémon species in Nintendo and Game Freaks Pokémon franchise, was designed by Atsuko Nishida, one of the character designers for Pocket Monsters Red and Blue, who based its design on Ivysaur, the mid-evolved form of Bulbasaur. Despite its English name, Ken Sugimori confirmed that the design of Bulbasaur and its evolutions are based on onions and bullfrogs. Here are some additional sources that support this:

• According to a Reddit post, Ken Sugimori based Bulbasaur off of frogs and onions.
• Some fans may argue that Bulbasaur is based on Beezlebufo ampinga, a massive prehistoric frog from Cretaceous Madagascar.
• Bulbasaur appears to be based on some form of Dicynodont, mammal-like reptiles from the Permian period, though it also has features from toads and frogs as well.
• According to a naturalist from Metro Parks - Central Ohio Park System, Bulbasaur is a combination of plant and animal, and appears to be some sort of frog with a budding leaf attached to its back.
• According to a post on PokéBase Pokémon Answers, Bulbasaur is based on a frog, and Atsuko Nishida designed Bulbasaur working backward from Venusaur.

In summary, Bulbasaurs design is based on a combination of onions, bullfrogs, and possibly prehistoric frogs and mammal-like reptiles.