What happens when someone attempts a knowledge check to gain information about a wholly new creature not seen before?
At first, I was thinking I would just use the Tarrasque as a basis. After all, it's a unique creature, and is even used as an example in the Knowledge skill description, having a DC of 15 + the monster's CR.
Except it also happens to be infamous. What are you supposed to do when a unique creature isn't infamous? What if it is something wholly new, never encountered before?
I would still allow a knowledge check, at a high DC (perhaps 25 + the monster's CR?), to allow the character to identify the creature's type and subtypes and all the features that type possesses. If they roll really high, they would also learn things about the creature, but rather having it stem from existing knowledge and lore, it would be represented by the character's immediate observations, astute assumptions, and knowledge of other, similar monsters.
In short not much would change short of the source of the knowledge in my games, but what do the rules actually say on the matter? How might you deal with such a situation?