Although modern man worships the legendaries, Arceus, or no gods at all, primitive man commonly worshiped gods modeled on other pokemon, most commonly Ivysaur. Some say this is because its blossoming flower represents adolescence, or because it unifies plant and animal in a single symbiotic form, or because its vines were seen as symbols of fertility.
All of this is true, but in reality Ivysaur were worshiped primarily because the plant on its back, in addition to Stun Spore, Poisonpowder, and Sleep Powder, can spray a powerful hallucinogen which causes man to have mystical experiences.
It is no coincidence that many prophets have had an Ivysaur in their lineup, and to this day they are commonly used in mystery religions and initiation rites – and not just among man. The mysterious garden where Bulbasaur evolve is filled with this unique Ivysaur spray.