What do you think is the logic behind Zubat’s Poison typing?
Well, for one thing, Game Freak seem to have disliked the idea of pure Flying-types up until generation V, when we got Tornadus, and it’s not really clear what else Zubat could possibly have been (no Dark type when he was introduced). For another, vampire bats are technically venomous by some definitions, since their saliva has special properties that inhibit clotting and increase blood flow to the area of a bite.
Pokémaniacal 