Why is this a problem? I don't need to know the particular sub-species of this dire wererhino to know that creatures that are covered in armoured plates can be stabbed in the eyes, or that you can use the weight of such a creature against them to wedge your blade in further...
The same thing exists with the Ranger's Favored Enemy. You might not know one dragon from another, but you know where to stab the lot of them so that they bleed the most.