Well, I think since the pet sort of has its life energies tied into the summoning ring, perhaps if it is killed in battle it just goes back into the ring and you can't call it out again for a longer period than if it just went back normally.
As an example:
You call out a pet and it can normally only stay summoned for 5 minutes, then it returns to the ring and you must wait another 5 minutes till you can summon it again.
But if it is killed in a battle, then it immediatly returns to the ring and you have to wait 15 minutes before it can be re-summoned.
or another idea is that to re-summon a killed pet then you need to use up a cetain item. For example since diamonds are used to make the ring in the first place, perhaps if a pet is killed then you need to use up one or two diamonds from your inventory to give your pet the power to return. Could make diamond mining a necessary job in Planeshift.