DC to know a monster exists?

[Ultrace]

I'm considering adding a custom item to my campaign that will allow the bearer to research information about monsters; when enough information is gathered, the item allows the character to transform into a (lesser) version of the creature for a limited time.

The question I have is: apart from actually encountering a monster previously, how do I determine if the character even knows a monster exists? Everyone has heard of dragons and vampires and probably trolls, sure, but what about a banshee? Or an ooze? Or a bulette? "Monster Lore" DC 10+CR covers identifying a monster, but that presumes it's right in front of you. What about just knowing of the monster's existence?

If the player themselves doesn't know about such creatures, but the character should, is there a certain DC knowledge check where I could just give them a bunch of monsters that they have heard of? This seems like an edge case, which might be why I haven't been able to find anything about it in the rules or other posts.

Thanks as always for any help or guidance.

[vhok]

My group does 10+cr and for every 5 you beat the dc by you learn another special trait about that monster

[Boomerang Nebula]

I go with the same as for identifying a monster:

DC = 5 + CR for common monsters (e.g goblins)
DC = 10 + CR for most monsters (e.g. bone devil)
DC = 15 + CR for very rare and unique monsters (e.g. tarrasque)

[avr]

If you simply want to know that say a carnivorous crystal exists that sounds relatively easy, but if you know nothing else about it how would you know to go researching it? I'd leave the DCs at vhok's or thereabouts.

[Azothath]

The knowledge checks are posted above.
This is really Homebrew (Advice). The PC should keep a list of known monsters and estimated Hit Die. This will help clarify it for the player and the GM. Things can be added (through some ritual) as you go.
Polymorph spell, which would seem to be the base of your item, requires an expendable material component, "a piece of the creature whose form you choose" which would imply there are a set number of creature forms in a given magic item. You should go with that restriction. Again , changing a form(s) in the magic item would require some magical crafting item rework or 'fixing'. If you wanted it 'loose' there could be a compartment which holds the material component and it could be swapped out as a full round action.

[ErichAD]

You know about trolls, vampires, and dragons due to legends and such. I'd continue on that theme for knowing about other creatures. Use either local or history to know what stories are told involving creatures, but I wouldn't expect to ever get from that starting point to real practical monster knowledge without first hand experience, an expert, a library, or a spell that gave you knowledge like Akashic communion.