They could learn:
1. That certain parts of the creature are valuable to the right buyer, delicious when cooked correctly or a potent aphrodisiac.
2. That they can not go near or are attracted to a certain substance.
3. That under the right conditions they can transform into another creature.
4. That they have some origin story related to a powerful entity or a force of nature.
5. That they are children/servitors of a larger, more legendary creature.
6. That some attack they do transfers a slow acting disease or curse, with a local cure (watch for the symptoms).
7. That they have some unique ritual associated with them.
8. That they have an odd reaction to a spell or item.
9. That among them is a shape-shifting caretaker.
10. That it's clearly a humanoid in a costume.
Most of these waste time after a fight or are quickly disproved by wasting a few action, an equivalent of one person being stunned 2. I based this list on folk stories, urban legends and fairy tales of common creatures.