The creatures in Cobra Bastion can be poisoned. They will then drop to 75% of their HP. That is the reason why you found them with "HP missing".
As you mention- there was none hunting around at the time you came to the spawn- it means that someone left the spawn before effect of the Flask with Snake poison expired. In this situation, two things might happen if you kill cobras:
In order to do that you need to have Empty Ritual Flask. This Item should be used on the fresh corpse of dead Cobra's changes into Flask with Snake Poison. Flask with Snake Poison can be used on Cobra Bastion on Large Cauldron (poisoning food of army in Bastion) which consequently has an impact on their life (they have reduced hp) for 30 minutes. If you planning to hunt there, it is recommended to have few ready flasks.
As item is not dropped by any monsters or received by doing quest and is spawned only once per SS.
Yes, you'll need to use a "Flask with Snake Poison" on the Large Cauldron inside Cobra Bastion, take in mind that the effect lasts for 30 minutes.
More details at Flask with Snake Poison
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