1) Describe creatures with appearance, not names. It helps prevent metagaming when dealing with creatures who aren't obviously something easy to recognize (i.e. dragons).
2) Common knowledge is whatever you deem it is in your game. For example, in my homebrew game dragons have been gone for 400 years, so I have made nothing common knowledge and all knowledge checks are 10 higher. If you rule that troll regeneration is not common knowledge, that is completely fine. Just stick with that ruling. Consistency is important.
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Also, remember that false knowledge can be more common than true knowledge.
To take a "real world" example, to kill a vampire, you need to:
Stake it
Cut off the head
Bury it under a cross roads
By exorcising the body
By recreating their original funeral
By filling the hole in the back of their neck with their hair.
(I seem to recall more, but these are the ones I could find fast)
Probably only a couple of these are true.
They still had bards and story-tellers and singers and such, though. For example, a lot of myths and fairy tales were oral, and only survive because someone wrote them down before they disappeared. We watch telly and listen to the radio. Pre-radio, everyone would to any (live) performances that were on.True, but how much of that relayed information is going to be accurate, as opposed to hyperbole rendered for entertainment value, assumptions, prejudices, or otherwise incorrect?
In an area where Trolls are uncommon, for example, but were known from long ago, there might be legends that "everyone knows" that say that Trolls can be driven away by dissonant music (just happened that the last troll seen in the area had a migraine that the time, and ran from a group of kids learning to play the recorder).
How common were the tales of Hercules in ancient Greece? How common was the Odyssey? There are tips for fighting the hydra and sirens in those tales.
In Ireland everyone knew that iron repelled fairies. That is why they put that iron horse shoe over the door. Lots of peasants carried a nail on their person to ward off fairy magic.
Also, remember that false knowledge can be more common than true knowledge.
To take a "real world" example, to kill a vampire, you need to:
Stake it
Cut off the head
Bury it under a cross roads
By exorcising the body
By recreating their original funeral
By filling the hole in the back of their neck with their hair.
(I seem to recall more, but these are the ones I could find fast)Probably only a couple of these are true.
Many years ago (playing AD&D) we encountered vampires and wound up trialling every rumour we could remember (I recall they all involved cutting the head off). There was something about lemons, and my Magic User summoned some, but the magic in the region was whacky, and she wound up buried under a cubic metre of them! Fun times :)