DinoInDisguise
A russian spy disguised as a t-rex.
That comes from the strange way that PCs do WAY more damage, while monsters have WAY more HP when compared to each other. PCs would slaughter each other in PvP. Monsters would slog away at each other.
This is an example of asymmetric design. The DM and the players play by fundamentally different rules. It doesn't actually matter if you scale HP on the monsters or scale damage. Monster design isn't bound by player character creation rules, and shouldn't be. WotC choosing the scale HP on monsters is a large part of why the monsters feel underpowered for their CR.
A monster can have 4 action surges, unlearn-able spells, impossible ability scores, and so on. The only limit is the self imposed encounter balance and, theoretically, complexity. Something too powerful will result in an unwinnable encounter, and something too complex will be functionally unusable. No other limits are on the DM.
This stems from the idea of balance between the players and the DM being impossible. The DM's power is infinite, the player's power is not. Any attempt at balance on this axis is futile by definition. And I would theorize that if achieved such balance would fundamentally change the game.