I'm passingly familiar with the psych literature on creativity, but the short review in the article reminded me that creativity is not just about making up crazy stuff (divergent skills), but also vetting the crazy ideas to make sure they serve your purpose (convergent skills). The convergent skills in particular are what makes the difference between "Yeah, she's very 'creative' (eyeroll)" and "Wow, she's really creative!"
Now that we're ready to move from the good side of the Fiend Folio to the bad, this theory can help explain bad monsters - in AD&D or anywhere else. Monsters can either fail to diverge, or converge. Converging is the more spectacular kind of failure, so let's cover failure to diverge first, AKA basic lack of creativity.
The prime example of this is where you take an existing monster, jack it up by a hit die or two, and pretty much call it a day. I'm not saying the Fiend Folio didn't have its flinds and what not, and even the Monster Manual needlessly promoted the otyugh to the, er, neo-otyugh. But the real champion of the phoned-in monster upgrade was Monster Manual II - greater basilisk, greater lammasu, annis, xaren, margoyle, storoper, thessalhydra, different colored slimes, jellies, puddings and oozes for Pete's sake ...
|Mmm. White pudding.|
Let's not forget the gratuitous monster breeding program, the kind that dares to envision the offspring of two critters already in the same niche - kind of like those towns where a Norwegian dating a German is considered "interracial." Thus, the FF's ogrillon, giant troll, two-headed troll (oh, they forgot the ogg-roll, which is what you get when you cross an ogre and a troll, right) ... yeah, and the gorilla bear.
Look, either you need a gorilla, or you need a bear. You are never going to need a gorilla bear.
What's sad about all these monsters is that they show so little faith in you, the DM, and your ability to add, subtract, or average basic numbers. If you want a super-tough ... let's call it a russet ... hulk, you should really be able to gin it up on the fly, adding some hit dice here, subtracting some AC there. AD&D is simple enough that you don't have to worry about his Spot skill or Charisma.
Next: The convergently challenged ... source of the truly legendary WTF's.