I have a mix of players both new and seasoned playing the Princes of Apocalypse campaign. They have played the Lost Mines of Phandelver and gotten to level 5 in that. I use miniatures when we play.
My problem is I am not sure how to reveal monster knowledge to them. Lost Mines had goblins, and hobgoblins and different types of common humanoid creatures. There were a few unordinary monsters and a few rare ones. So far I have gone with the assumption that most adventurers would recognize the most plentiful types monsters (Orcs, Goblins, Hobgoblins) from stories of the wars with them. They would probably know what some of the fantastical ones (like Dragons or a Medusa) would look like too, from being told tales about them. But the characters cannot have heard about EVERY creature that exhists! So when they meet something like a Umber Hulk or an Ankheg for the first time, what am I supposed to do? I have tried to let them do nature checks, or even arcane checks on magical creatures. But then if one char fails the ID, the next char tries. With 5 players, they usually get the roll. And then once they know the creature's ID, I've given them the types of attacks and weapons to expect from the creature. They then use that info to plan their assault.
I'm not nessesarily tryng to prevent an identification of their enemy, but I think there should be a balance of what they know and don't. Is this the kind of thing I should use a group check on? Should I create a DC based on how much of a challenge a creature is? Any advice or systems that you use?
I wrote a guide for this sort of thing. Check my signature for "How to Adjudicate Actions." There you'll find examples of how to handle these sorts of interactions as well as others.
My suggestion is remember that players control how their characters think and that can include stuff they know. So if an experienced player says his or her character knows about umber hulks or ankhegs, then that's okay. In fact, you might want to ask them to establish how his or her character knows that. In the doing, everyone at the table gets to learn something new about the character, and that's a good thing. It adds depth to the character and to the setting.
Of course, there is a danger inherent in this approach: A player may make assumptions that turn out not to be true! For example, perhaps you've changed up that ankheg so that it sprays fire instead of acid. A player is going to be very disappointed when he or she has his or her character drink a potion of acid resistance only to be bathed in the beast's flaming breath. So I would advise that you tell your players it's okay if they establish things their characters know, but that it's wise to take in-game actions to verify their assumptions so they don't get burned!
When a player decides to have his or her character try to recall lore about a monster, you have to decide if that action is successful (no roll), fails (no roll), or whether that action has an uncertain outcome (roll). In the case of the latter, consider using "progress combined with a setback" on a failed check. For example, if the new player wants to see if his or her character can recall lore about ankhegs and you think they have an uncertain chance of success, ask for the Intelligence (Nature) check. If the check is successful, provide the player with whatever knowledge he or she seeks. On a failed check, give the player the same information, but at a cost. Perhaps the character hesitates and the monster gets an attack against PC as a reaction or the sudden terrible revelation of what the creature can do make the PC frightened until the end of his or her next turn. By providing the information on a failed check, you stop the cycle of everyone wanting to roll until somebody succeeds.
I find it rather lame if players know the exact stats a monster has and the exact attacks a monster can do. Even if they roll a 20 on a nature check I wouldn't tell them that.
What I do:
1. First of all, if the enemy they encounter is far stronger than them and they can't possibly defeat it just yet, I kind of let that flow into the description of the scene.
"Just looking at this creature gives you the chills."
"Your confidence in defeating all enemies in your way quickly fades as this creature draws closer."
I also tell players any other obvious information that doesn't require any rolls, but might be relevant (like how the skin looks like or that the creature seems to avoid light sources, etc.).
2. I allow players to recall stuff by doing Nature checks. Luckily in my group, players automatically don't want to do the same actions each. If only one tries to recall and then tell the others if he knows something important, that gives a much better flow, than if all roll, potentially all know something different and then each one gets to decide how much he tells the others. Not to mention that it becomes impossible to overview who knows what.
If I had such a group, I'd probably add some special handling like "All roll at once, the one with the highest result gets the information and then can tell the others".
Information received like that still doesn't involve stats or attacks and directly refers to what the player wants to recall:
Player: "I want to recall what I know about goblin dens, before entering it."
DM (bad roll): "You remember something once having told you that goblins like dark places and that their dens are pretty dirty."
DM (good roll): "You remember that goblins like dark places, because they can see pretty well in the dark and are fond to place traps to defend their dens."
or
Player: "I want to recall if this creature has any weakness."
DM (bad roll): "You have never heard of this creature before, so you can't know about it weaknesses. Its skin seems pretty hard, though."
DM (good roll): "You've read in a book that these creatures are quite resilient to physical attack, but fear fire."
Generally, there is usually also information you can provide on a really bad roll. It might not be useful, though. I often even scale it depending on the result instead of just having "knows or doesn't know" like:
0-9: Some useless information.
10-14: Some hint that might help if interpreted correctly.
15-19: Hint plus additional useful info.
20+: All previous pieces of information plus a very useful clue.
The goblin den question is a good example. You have plenty of different information you can share:
a) Goblins like dark places (players can conclude that they have darkvision from that)
b) Goblins are fond to place traps (players can conclude, that they should be careful to not step in one)
c) Goblins like to gather in larger groups (players can conclude, that there are probably several goblins inside, if the den is still inhabited)
It all gives the players useful information, but without telling them anything about the goblin stats at all.
I often ask my players if they would know the creature, unless it makes sense for their class to have that knowledge. If the player has a good reason why his character might know the creature, then I allow it, and they don't have to roll anything. Otherwise it really depends on their class:
Druids and rangers: I allow them a knowledge nature roll for animals, such as an Ankheg. But also against fey.
Clerics and paladins: I allow them a knowledge religeon against any undead or demonic creatures.
Sorcerers and wizards: I allow them a knowledge arcana against magical constructs, and other magical creatures, such as dragons.
Bards: I allow them a bardic knowledge against mythical creatures, such as dragons.
Working out what the party thinks a creature's abilities are seems like a good time for a group check as members of the party insist that medusae can be defeated by throwing odd socks on the floor and slaying them while they try to find a pair.
I'm really not a fan of iserith's "random barely connected thing happens to punish you for failing a roll" method. I think it's fine for things that are actually actions, because you can create a logical outcome, but it breaks down and feels nonsensical when applied to fundamentally passive things like knowledge checks and perception checks.
1) The description that you give of how the creature looks, sounds, smells, acts can convey a lot of information. Ryo.Reisender did a better job of an example of this than I can right now. 
I consider this the best approach for a new creature. Show don't tell.
2) If it's important to the scenario you are running, you could consider one of the following sources of info as appropriate: baking some lore into an encounter (the PCs discover a written account of the creature), in-character exposition (the frightened peasants describe the monster to the PCs) or first-hand observation (the PCs arrive on the scene as the monster attacks. They get to see its attacks and abilities being put to use). I'm sure there are many more!
I'm not really a fan of knowledge rolls for the reasons that you describe. If I consider it common knowledge in the game world I just wave it and tell the PCs what they know. If it's common player knowledge (e.g. trolls are resistant to fire) I normally wave it as common character knowledge.
You can and should decide on a case-by-case basis what is important. For example, I wouldn't let the PCs blunder into an encounter with an enemy they had no way of defeating without imparting some information to that effect.
Here's how I decide what the characters know about a creature:
If a creature is something that maybe the characters have heard of it because of some book they read, encountering before because it's fairly common, hearing tales told about the creature, or what have you: I tell the players what the creature is and what they need to know to make informed choices while fighting it. Examples: goblins, dragons, even most planar monsters, I just say "it's [insert monster type], you can decide how your character knows that, or doesn't, as you wish."
The only time I don't tell them what a creature is and what they need to know to not make mistakes while fighting it is when the point of the encounter is that the party is facing a creature that no one has ever heard of before.
Example: In one campaign, the central events follow a character who has been sent from one planet (home-brewed campaign world) to another in a different star system (Mystara) on a mission to strengthen herself, make allies, and accumulate power and resources to aid in a war for the survival of her home world. When this character started to encounter creatures that aren't native to her homeworld, she knew nothing but what she could see - it looked like she was facing hobgoblins, but their skin color was not what she expected. That detail wasn't too surprising to her because she had already faced goblins that appeared in every way to be goblins just like from her home world except that their skin was earthy tan instead of grey. However, when their attacked with claws that felt like the touch of a ghoul, a sensation she is familiar with, she started to wonder what these creatures actually were - and that they possessed regeneration began to worry her.
After the encounter, her companion native to Mystara pointed out that such creatures (looks like a hobgoblin, regenerates like a troll, but otherwise is basically a non-undead ghoul) are called Thoul, and they are nothing out of the ordinary for this world.
I just don't feel like creating a "roll gate" between info and being able to actually use said info adds anything to the game - but maybe that is just all these years feeling like the only character I could actually play in any other DM's campaign was one that had strong modifiers in every knowledge skill so that my knowledge of the game from being a DM wasn't being viewed as some kind of "cheat."
Simple method of resolving the "everybody tries the knowledge check" issue.
Tell the table one - and only one - roll can be made to see if the group knows anything about the creature. Let them decide who will make the roll.. If they can come up with a good reason one of them might have special insight into the question, give them a bonus or advantage.
If they make the roll, the characters know what the players know. If they don't, it's up to them how to react. You can reward inspiration diice for what you feel is appropriate behaviour in that case.
Also, change the monsters up occasionally. Fireproof mummies who freeze easily, Umber Hulks whose eyes do nothing special but whose claws exude confusion causing poison, Trolls who gain a hitdice from fire but take 3d6 from a bucket of water and can't regenerate.
If the players make their rolls, they know these things. If they get lazy or just make assumptions...
Don't sweat it.
How many animals can you name? Mammals? Insects?
How many countries? Foods? Sports teams? Can you name sports teams that you've never seen before? How about their mascots?
How many singers and performers? How many of those that you've never seen or heard perform?
People share info, tell tall tales, regale one another with stories. Every people has a popular culture. It isn't a stretch to think adventurers would know about things they might encounter while adventuring.
Rely on these expectations. Rely on the familiar. That way, when you really want something unique and unfamiliar, you can turn expectations on their head for some Real Fun.
Otherwise, who cares? Is this knowledge adversely impacting your encounter difficulty? We can always write different encounters.
Thank you for the advice, everyone. I think I will improve my descriptions of the creature/foes and then add in the checks if the characters wish to use an action to recall more helpful information about the enemy. I apprciate everyone's responses.
Although it is true that normal people in a fantasy world would have heard of many monsters, I think setting both in regards to which gameworld, as well as what part of the world (cosmopolitan, points of light, etc), would dictate a great deal of this knowledge. Heard of goblins, hobgoblins, and kobolds, but never seen one? Might take a good look to see. Also . . . a 1st level PC is essentialy a noob, at least in my games. 1st level PCs with tales of grandeur about long extensive adventures woul mean they aren't 1st level, and are not allowed in my campaigns, unless they are NPCs in a PC's backstory, in which case the NPC isn't run by the player anyhow.
I as well, being a DM, know about most monsters on the fly, but I always hold back unless I make some sort of roll, to be fair.
I go with a house rule similar to Pathfinder.
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identify traits about aberrations, dragons, elementals, monstrosities, and oozes (DC 10 + CR, every 5 points over an additional fact is learned)
SadisticLeprechaun wrote:
HMmm! That is an interesting thought there, maybe I will! It might give players a chance to get more details about monsters as well since the DC is 5 lower . . . ok I will give it a whirl next session, and see if people suddenly always can tell, or if there are still times they do not know. Thanks!
This sort of thing became an issue in my game a little while ago. One player shared information about a monster that another player did not think would have been known by the characters. We had a discussion and decided to allow the 'metagame' information to be shared but it was up to each player to decide if his character knew it and how to respond. It was emphasised that if a character made sub-optimal choices, or ignored the advice for rople playing purposes, the other players would not criticise them for doing so.
One player suggested characters should know almost all monster information as it would have been recorded by adventurers over time, compiled, and made available in the in-game-world version of the MM. I wasn't convinced; not because of the logic but because of the size of a book that contained that sort of information, given there are no small-font printing presses in my world.
If identifying the monster makes the encounter boring, it doesn't say much for the encounter, but if mystery is needed to, for example lend a spark of horror into the game, I often customize the monster and make it hard to see. I have a DM who plays, and he can identify a monster and still be afraid that it is too powerful (he doesn't remember CRs), or that it isn't what he thinks it is (given my customizations).
When applicable, you should warn players upfront that you don't stick to MM alignments, because the players immediately identifying a monster the PCs've never seen before as evil and needing to be attacked on-site, when your version is quite undeserving of such treatment, can take some fun possibilities out of the game.
In typical weird fiction style, I almost never name monsters, instead giving epithetical descriptions. My monsters have different cultures just as humans in the campaign do, and they don't keep the same appearance across all my settings (but are consistent within a campaign). To wit: orcs in my games have been called "dark, tusked men", "boars on two legs", "pigfolk" and "the chuckling, tusked men you saw earlier attacking the fort", and one nation of orcs isn't the same as the next, and I've told my I don't stick to default alignments.
Rolls to identify things: I roll and keep the result a secret, spinning short tales their PCs heard from the lips pf travelers of the shaggy beasts of the moor who move much faster than their size would bely, or the song scrap remembered of a man who was chopped up and put together again, but was ever afraid of a candle. I haven't given them false info before on failures. If I did, I'd add more uncertainty to the tale if it were a lie, and be clear how reality contradicts it when tested.
AaronOfBarbaria wrote:
- Don't hand out the creature's game stats, but DO read the flavor text.
- You can let them estimate the power of its attacks by comparing their character's Hit Dice to the creature's Damage Dice. (So the Paladin doesn't worry about getting 'a little scratch' from a d6+2 Claw, but the Wizard stays way the heck away from it - you want this anyways.)
- Keep an adventure log so the group can do a 'memory check' - Didn't we fight one of these last month? - to avoid extra rolls.
- If not using methods described in posts above, have ONE person make the Knowledge roll, with ONE other person assisting; the helper either permits Advantage on the roll or gives a +2 bonus on the result.
- Clever players might keep trophies from their victories. (How MANY characters want to craft DragonScale Armor?) This too is an aid to memory. It can also be a bonus to Intimidate checks - or it might ensure Intimidate(epic fail) results: "That was my cousin Benny, I'd recognize that scar anywhere !"