One of the things I’ve always had an issue with is within all versions of D&D is the idea that everything is a “Monster”. Many monsters in the Monster Manual are unique and would not be encountered more than once in a player’s lifetime. Bosses like Smaug the Dragon or Orcus the Demon Prince should be epic creatures. These are not stat blocks, these are very special, very powerful, and very one-of-a-kind. They should be removed and reframed as NPCs or Monster Bosses, not just XP dumps.
Here are just a few monsters who should be more fleshed out before they are inserted into an adventure.
Dragons
Yes, young dragons or drakes would be much more common and should be listed in the Monster Manual, but older dragons would be very rare indeed! They should be treated as NPCs in their rights and not just stat blocks. For example, In the book, The Hobbit, the main dragon is sometimes called Smaug the Golden or Smaug the Magnificent, bringing ideas of a unique monster who’s lived for centuries and has a substantial reputation.
Devils / Angels
Devils and Angels are other very limited creatures. There are low-level monsters, but the big ones – the Arch-Angels or Demons Lords are special, each with their backstory and supporters.
For example, Orcus is the fictional demon prince and lord of the undead in many campaign settings for the Dungeons & Dragons fantasy role-playing game. He is named after Orcus of Roman mythology. His symbol is a mace with a human skull as the head. Orcus is one of the most detailed demon lords of the Dungeons & Dragons game and one of a small handful to be detailed in every edition of the game. Orcus was also named as one of the greatest villains in D&D history by the final print issue of Dragon. There is only one ORCUS. One. Ever. He should be more than a single page of stats. He has a cult, an organization, superpowers, AND a crazy will to survive.
Liches
High-level magic-user turned undead? Cleric who’s cheated death by becoming the undead themselves? Another monster is a unique NPC. They should be played with personality, creativity, and, above all, a willingness to negotiate or run away for revenge on the players.
NPC Monsters And Flavor
The list goes on and on. There are plenty of monsters who are fodder for players – adding a nice unique monster boss to an adventure can add even more flavor. In our games, there is a recurring Magic User who is always behind the monster attacks and conspiracies. He always seems to escape just before the PCs have the chance to kill him. They’ve been following clues and rumors trying to end his terror spree.