Bulbasaur is slightly curious, no? Why do you think the devs decided to give a starting Pokemon a dual type? It risked confusing new players, and there’s nothing about the design that particularly screams “Poison-type” anyway.
Ehhhh I don’t know about that. Poison has no interactions with Fire and Water, so it doesn’t mess with the basic starter trio at all, which is how they teach type interactions, and new players will meet dual-type Pokémon on literally the first route anyway (Pidgey is Normal/Flying). It’s true that Bulbasaur doesn’t really need to be Poison-type, and I’m tempted to put that down to Game Freak not having finished figuring out what Grass-types actually were yet. The only poisonous-sounding thing Bulbasaur can do is powder attacks, and Tangela, generation I’s lone pure Grass-type, actually gets a wider variety of those (Bulbasaur misses Stun Spore). So my best guess is… Bulbasaur is conspicuously one of only two first-generation Grass-types that are part animal – the other being Paras, whose hybrid nature is very clearly called out by his Bug/Grass type. Maybe back when Pokémon was first getting started, they had a feeling that plant-animal hybrids couldn’t be pure Grass, and then just latched onto Poison as the secondary type that seemed most appropriate to a frog-like critter like Bulbasaur? Obviously, if that ever was a real design principle, it’s one they abandoned in the very next set of games with Chikorita (who also learns Poisonpowder), but I think if we were still in generation I, with the 150 original Pokémon, there would be an argument here for making Bulbasaur Grass/Normal.