At the moment we have 3 types of every monster type, see (consumer, neoten consumer, imago consumer). They have a certain % of different stats already, and they vary in size/texture. Making a pack of monsters should then include all 3 types in different %. That's something we can already do today.
Another idea (somehow expressed in this thread) is to have the same monster and same type vary in stats slightly, like +/- 10%, this is surely realistic, but I'm not sure it will be that noticeable. So for now we go with our 3 variations per monster type.