Guys I think after 25 years I FINALLY just understood why Zubat and Golbat are Poison Type. It's because vampyrism is a magical disease and Poison Type is associated with infection. It's not healthy to be a vampyre, it's a horrible affliction and you could corrupt someone else with your bite.
The vampyre disease is a contamination. That's why they're Poison Type, not unlike how others of that Type evolve to have it because they live around contamination of other kinds.
