Disclaimer: I don't have a fully developed beetle that I use often and am happy with the template.
For my use case, my use of poisoning "Magical Ability" is on an Anniversary Horse. I use the 3 slot pet to cull nightmares for rare spawn ones, and on Siege to pull Fire Steeds from the Deceit brazier. For nightmare culling it's good because it causes the nightmare to focus on curing rather than casting. For Siege Deceit brazier, it's effective on clearing out the other stuff that spawns, I can still honor tame the Fire Steed, and if I get PK attempted, the extra type of attack (Poison/Bleed/AI pet template) adds pressure so I can get off a gate outside the dungeon if they run off screen to heal. It's a contained space so a 3 slot horse getting off Bleed, AI, and Poison plus melee attack adds pressure. If you had offensive capabilities or a mage weapon you might be able to seal the deal on hobbyist level solo pks. Siege pro level pks will usually kill you too fast (I ping 200+ at best), but they usually just harvest your gold and bag of sending if you have one. If they are a bit mental (happens with pk types) they will take other loot to grief, but your time to pull from the brazier is quick and if you kill off the spawn there isn't much to show your activity. Some pks actually like a good fight back and will shoot the breeze with you afterwards.
So... yeah, tldr. Very limited Use Cases for Poisoning as a Pet Magical Ability, in my experience.