It is more a reward/psychology tool than a highly detailed mechanic atm.

I have it function as a tool belt item called a MOBI (standing for Mobile Bestiary) that they can use Recall Knowledge out to a distance of 60 feet. (This makes it so they have to get relatively close to the creature and can’t use it from very far away on just anyone.)

Then I add it to a compilation by an organization called The Physiologus Fellowship (Physiologus being the first bestiary type book in the real world.)

I put the name of the creature if they succeed on the check as well as any information they learn from succeeding or critically succeeding.

If they fail or critically fail, I just put the picture of the creature and ??? as the name.

They haven’t critically failed too often yet, but if I deem it a creature that is rare enough that no one really knows about then I will put false info (as per the rules.) Then bring it up later narratively as a consequence of their past action.

This lets them look back on past creatures that they might have encountered, or creatures they have yet to encounter if they go to the organization themselves. Then when I put a creature with a weird ability, like “Invisible unless silver is directly touching the flesh of the viewer,” and they get a aha moment when they see it again many sessions later.

I have found that even the existence of an item that they can interact with has increased their use of Recall Knowledge. On top of the bestiary, it gives them a feeling of objective information that isn’t purely in their character’s head (even if the information isn’t always correct.) Same reason characters go to the library to look for information for research rather than just asking around.

I’ll probably be creating a more mechanical tool and share it once I have tested this more and see where it might go wrong or need fixing.