Ways to Switch Up D&D Monsters

I've been having a blast reading Descent into Avernus, the first real D&D 5e book that I honest-to-goodness bought not just for access to new game mechanics, but to have as an actual adventure module to run. One thing I admire about Wizards of the Coast's published adventures is their ability to breathe life into combat so that it always feels new. They always seem to give great variety to the types of monsters you encounter in an adventure; for example, you don't just see devil after devil when you go to the Nine Hells; you also see a gang of wereboars, a night hag with a crew of fey redcaps and a pet flesh golem, and a chariot pulled by gorgons.

Even with the devils they do put into this adventure, they use simple twists to make each one seem unique from the standard picture shown in the Monster Manual. Below is a list I've compiled showing some techniques and twists you can put on monsters (along with examples) so that players who are already familiar with all the monsters in the game can encounter new challenges, and finding yet another group of orcs won't ever get burdensome.

And because this is D&D, I've made each table one you can roll on if you need an idea in a pinch.

Monster Variations

d20 Variation

1 Gender swap (female planetar, male medusa, manticore mother with cubs, female ogre)

2 Injury (missing a limb, disfiguring scar, limp, eye patch)

3 Mutation (ettercap with extra arms, two-headed hippogriff, scaly mind flayer)

4 Alignment change (brain injury or augmentation, outlier among others, repentant, friendly)

5 Individualist (pacifist, shy, cowardly, other agenda, jealous of allies)

6 Template (skeletal roc, half-dragon ogre, zombie rats, golem horse)

7 Noncombatant (troll children, elderly bugbears, lame or sick hobgoblins)

8 Weapon swap (goblins with pikes, death knight with bow, ghouls with slings)

9 Character levels (orc with sneak attack, wyvern with second wind, kobold with rage) 

10 Mounted (cloud giant on a roc, skeletons on skeletal warhorses, pixies on faerie dragons)

11 Paragon (multiple enemies mashed into one) (medusa hydra, animated armor-wearing jackalwere)

12 Progressive (flying basilisk that loses its wings when reduced to half hit points, bulette whose armor sloughs off over time, berserk trolls)

13 Damage swap (flying hammers, ice imp, radiant wraith, poison gas elemental)

14 Spell swap (archmage-archpriest, apprentice cleric, lizardfolk shaman-warlock)
15 Armor swap (goblins in full plate armor, armored wyverns, blackguard in black studded leather armor)

16 Reskinned (kuo-toa murlocs, red dragon phoenix, flying books (bats))
17 Magic item equipped (ogre with headband of intellect, beholder with ring on eye stalk, gnolls that drink potions of healing)
18 Age/size change (sphinx cub, ancient ankheg, beetle-sized gelatinous cubes, gigantic intelect devourer)
19 Legendary (legendary actions, resistance, powerful reactions)
20  Ability score swap (intelligent ettin, sickly giant, fast treant)

Enemy Group Variations

d12 Variation

1 A large number of "fodder" enemies. Same stats except only 1 hit point

2 Enemies that ambush the PCs

3 Enemies that set a cunning trap for the PCs

4 A second wave of enemies after the first (reinforcements)

5 Two separate factions of enemies in one battle (different colors)

6 Enemies who are out of reach and attacking with ranged weapons
7 Enemies disguised as other enemies

8 Illusory enemies set up by the real ones

9 Enemies who retreat or surrender immediately

10 Enemies led by a boss (boss has maximized hit points)

11 Enemies with visible leverage (hostages, destructible treasure, a lit torch in a wooden house, support columns underground)

12 Enemies arguing with each other who automatically fail Perception checks for the sneaking PCs