Looking back at the original 15, some specific questions that come to mind...
- What niche did gnolls fill in earlier editions that wasn't filled by goblins, hobgoblins, ogres, or orcs? (In more recent editions it seems to be the "bloodthirsty" foe, but that identity seems to be a 21st century invention.)
- Red and white dragons are understandable (fire and ice), but why is acid (black dragons) the third pillar?
- Are gargoyles the fundamental "flying" foe?
- Gnolls and kobolds (in their D&D incarnation) don't really seem to have a clear Tolkien/mythology background like the other four humanoids. Is there some forebear I'm missing?