Lets take a look at the names:
Bulbasaur: Bulb and saur, saur meaning reptile
Ivysaur: Ivy and saur
Venusaur: Venus for venus fly trap or Venom for poison, and saur
So its a reptile, but not only for "saur" at the end of its name. Many pokedex entries state the hours long it stays in the sun taking in sunlight. This means its unlikley its a frog, as they need to be constantly humid. This is also a caracteristic of lizards, as they bathe in sunlight because they are cold bloded and need the sun for heat. lastly, bulbasaur red and blue dex description:"A strange seed was planted on its back at birth. The plant sprouts and grows with this Pokémon." With this we can conclude the bulbasaur line is a reptile living in symbiosis with a plant.
But this brings up another question. What exactly was bulbasaur before the seed was planted? What happens if a bulbasaur dosent get a seed upon birth?
My best awnser is helioptile. There arent exactly tons of lizards but two things make helio more probable:
1:They also absorb solar energy, exept to produce electricity.
2:They are slightly bipedal, and bulbasaur can stand on two legs, because:
Ivysaur red and blue dex entry:"When the bulb on its back grows large, it appears to lose the ability to stand on its hind legs."
And implies that bulbasaur can stand on its hind legs.