In some ways it's a general problem with "Monster Manuals" and the like: if the GM can read about some beastie, then so can the players. Designers come up with variations in an attempt to keep critters fresh, but eventually the players learn how to beat those too.
Originally Posted by Thoth-Amon
A better solution would be "monster generators". Ideally a GM should be able to design a creature from scratch in very little time. However, built-in mechanisms to customize existing creatures, such as Chaos mutations in Warhammer and RuneQuest, would also work. Wizards are the ultimate monsters in that respect: there's no telling what spells they have prepared (unless they're 4e PC classes).
Of course, an easier solution is to reuse the same "monsters" ... who are intelligent, versatile, and can use tactics. In a Midnight campaign most of our adversaries were goblins, orcs, and legates, who had their own goals and agendas, and who used their environment and tactical advantages to the fullest.
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)