Monsters should be common, uncommon, and rare too, like magic items. Or maybe famous, obscure, and hermetic.
Famous monsters have a knowledge DC of 5. Obscure of 20. Hermetic of 35. If you make your check, you know the basics about the monster. If you beat the DC by 5 or more, you know just about everything about it.
For instance, orcs are famous, as are dragons. Everyone has heard stories about dragons, so most everyone knows the basic delineations of colors and how they match to elements. A specific dragon, though, might be obscure if he's not been active for a while. And an elder dragon who has lain dormant for millennia would be hermetic.