Bane Dragons and Wildfire Ostards have passive Poison skill, which does not raise unless you swap their Magic to Poison or give them Poison Breath. No real point in doing that though, since they innately have Poison Skin, which will inflict Deadly Poison on melee attackers if fed Blackrock Stew within the past 24 hours.
I used to farm Raptor Teeth with my Tamer/Mage, and would gather up a ton of Raptors, invis, hop off my Bane Dragon, run Consume Damage on him, and laugh as all the Raptors would quickly become DPed attacking him.
Best Poison pet though, is probably the Najasaurus. With Poison+PB+FWW, they have a ton of AoE damage output (the highest AoE of any pet), and can tank most spawns pretty well (especially with Consume Damage) due to high overall stats (they based the Triton stats off of Najasaurus), way overcap Resist skill, and complete Poison status immunity (can always be healed unless under Mortal Strike).