I find it trivially easy to imagine how a novice character might have knowledge a veteran player would have. An acolyte might remember it from an obscure sermon he or she once heard. A criminal could have gotten the information from his or her underworld contact. A folk hero might be destined to defeat such a creature and so has been steeped in folklore since birth. A noble might have had access to histories of his or her house which included a confrontation with such monsters before. A sage could have read it in a book. And so on.
Absolutely. Many of these monsters are based on the tales and lore of past ages anyway. In other words, they were tales and stories known (and often believed to be true) by a large part of the population. It just so happens that in D&D, they are true. In our world, for somebody in medieval France to not hear the German tales of trolls is a cultural thing. In the Forgotten Realms, it could be a deadly thing.
More importantly, the fantasy D&D worlds have some significant differences with our historical world. In an age before TV and even, for the common person, books, stories are entertainment. This is a world where dragons, trolls, and worse are real. Every single village has had some threat from monsters, undead, and orcs, goblinkin, ogres, and the like. Bards not only entertain, but record the history of a magical land.
A novice character is a novice in their class skills. They are not, however, a novice person living in a land and frankly, even a young 17-year-old character knows more about their own world than a veteran player could ever know. Simply because it's the world they live in. But we're also talking about a world where a given party may consist of elves, dwarves, humans, and other races. And what the elves teach their young is different than what the humans teach. The knowledge of their elders that are hundreds of years old may not only have heard the bards' tales, but experienced every such creature in person.
For example, in the Forgotten Realms, past supplements and novels have identified cheeses, ales, wines, favorite recipes, trees, common plants, and all sorts of other lore. Then there is the endless amount of lore that has never been published. When your character walks down their home village or town street, they know people by sight, who lives there, and the tales of the ruined keep in the woods. Things that the DM may not have detailed yet, or even knows existed (because nobody has explored that part of the forest yet, and the random roll that places the ruins hasn't occurred yet. Of all the stuff that has been detailed about any fantasy world, the actual characters know more.
When a mother and father tell their children that the silver arrows in the corner are to be used only against werewolves and other were-creatures, it's not a myth or folklore.
While it's nice to role-play the elf telling the human something new about a creature they have just spotted, we don't really have the time to play out the character's lives in real time. So much of that is glossed over as knowledge that is shared at a meal at a campfire, or over an ale at a tavern.
Travel and trade in the Forgotten Realms is much greater than it was in our own middle ages. The Realms (and most fantasy settings) are not feudal systems, and they are not typically embroiled in the same constant war between states nor anything like the crusades. News travels with the trade, and again, has for thousands of years.
When people have been fighting these monsters for literally thousands of years, there is very little of the basic folklore that I think would be unknown. I alluded to it earlier, but in my campaign, pretty much any surface creature, monster, humanoid, etc. that is known to be a potential danger is known by at least the basics. That is, use fire against a troll (maybe not acid, that's a bit rarer to have on hand), silver against lycanthropes, undead can drain your strength (hit points, etc.).
Sure, they may not be able to tell the difference between a troll and an ogre on sight, or a wight and a zombie. Which means they will treat them with more respect, and might attack an ogre with fire just to be safe. People in the North probably don't know what a wemic is, because it's so far removed from their homeland in the Shaar. But they know what a centaur is, and would probably equate it to that until they knew more.
I don't typically use the art in the MM, mostly because so little of it reflects what I think the creatures should look like. So I give descriptions, and a walking corpse is a walking corpse of some sort. Could be a zombie, could be much worse. The uncertainty of knowing what something is, rather than trying to determine what a character would know once identified, is a much better approach. Because the player and the character are both suffering the same problem - they don't know what they're up against.
As I said, sometimes my players will decide there is something the characters don't know, even though the players do. That's tough to play out though. Would they figure it out? How long to take to figure it out? Etc. Instead, if there's something where it's actually important that they don't know it, then I'd agree with what others have said. Change it.
Try a trivia game to see how much "useless" information you actually know. Then imagine if even 25% of that useless information was actually about real monsters that might use real magic and might attack your village on any given night.
