Although Bulbasaurs already exist since the Permian period, the first real Bulbasaur was to be found in Pallet Town, under the ownership of Professor Oak's great grandfather back in 17th century. It was found near a water-lily pond where he accidentally falls down to the pond before facing with Bulbasaur and decided to keep it. Because of the not too smart people we have at that time, many people who sees Bulbasaur called it a 'monster' despite its small size. Why? Do you ever see a toad-like creature with mysterious bulb on its back? No, right? However, Oak's great grandfather took great care of it not just because of its cuteness, but its extreme loyalty to him. Since then, Oak's great grandfather has gained a large popularity from owning such a rare creature.
In modern days, Bulbasaur were still considered as a rare Pokémon, since only few people were lucky enough to spot Bulbasaur in the wild. This low population has forced Pokémon Breeders inducing Bulbasaur to mate at early ages in order to control the population. An alternative way suggested by Professor Oak is to give Bulbasaur as a starter Pokemon to new Trainers and let them train the Pokemon. This method proved to be working as the number of Bulbasaurs in the wild were slowly increasing.
Interesting Facts:
Bulbasaur gets its name from the bulb on its back. And, do you know why Bulbasaur has dark patches on some parts of its body? Blame Professor Oak for this when he is conducting an experiment to find a way of keeping Pokemon in smaller object known as Poke Ball. His unsuccessful attempts hit parts of Bulbasaur's body, forming a bruise. Eventually, the bruise heals but into dark patches and this has becomes part of Bulbasaur's genes, which is why, you bred this Pokemon and the offspring still have the patches. Bad Professor Oak. :P