In my own campaign world, I do this all the time. I have dozens of types of skeletons and zombies; jungle stirges; arctic owlbears; a score of poisonous snakes (based on real life examples like the black mamba and fer de lance) each whose poison has differing effects; various varieties of iron cobras (some giant sized), spell-casting ogres, and much more. I know there is nothing "original" about this, but I have always disliked the player who memorized the Monster Manual to the point of a rabid zoologist and always knew the EXACT spell to counter any monster (which is the best time to mention that a blessed crossbow bolt does SQUAT to rakshasas in my world.....!) When I created my own campaign world, I gave certain monsters the "week off" (there are very few hobgoblins, bugbears and drow) and instead concentrated on more varied undead, climatical varieties of regular creatures (arctic owlbears and snakes, jungle stirges and ogres, desert spiders, swamp landsharks, etc), and intelligent spellcasting gargoyles, spectres and dopplegangers. I also allow monsters to pick up and use items like any other schlub. Why wouldn't a halfway intelligent creature pick up that glowing sword or shield instead of leaving it sitting on that sack of gold? The dead adventurer was wearing this pretty ring? I'm putting it on...hey, I'm invisible!
Confounding player's expectations is really doing them (and you) a favor. If a character in my campaign sees a giant of any type and assumes ANYTHING other than it looks mean, powerful, and could have something up it's sleeve this side of a vorpal blade, they have only themselves to blame when the "simple" hill giant begins beating the tar out of them using a girdle of Storm Giant strength...or begins casting a fireball at the party standing out of missile weapon range. Presuming to know the DM's world (or mind) without empirical evidence can get you dead really fast in my campaigns.
One of my favorite encounters was many years ago in some campaign or another I ran, as a band of adventurers was trudging across the wilderness and came across a kobold sitting on a fence post. He eyed the approaching group but did nothing to move or hide (a warning right there). In the middle of nowhere, the heavily scarred critter exuded toughness and had a gleaming sword hanging on a scabbard at it's side. Veterans of my campaigns knew something was up, so they gave the grizzled warrior a nod and wave and continued on. Newbies were looking at the vets with mouth agape..."It's a kobold, for crying out loud! Free XP! Let's get 'em!" as their characters rushed to what they thought would be easy pickens.
A few rounds later, the 10th level kobold warrior having viciously thrashed the sadder but wiser newbies (being careful not to kill them lest he raise the ire of the other adventurers), he paused to spit on the ground and pointed to the pile of groaning bodies. "These idiots belong to you?" he snarled. The unbeaten party members smiled, shrugged and nodded as he strolled on, his rest spoiled. The players learned a valuable lesson that served them well: appearances can be deceiving, and don't base your expectations on what is in-between the covers of the MM. They soon learned that while the MM provides a baseline description, in my campaign world, it pays to be cautious.
Here is a very heavily edited chart I sometimes use when I want to spring a little surprise using a iconic D&D critter. I usually just use it when I'm working on a unique encounter or trying to create a "boss" type with a little more "oomph". It's not the quality of a James Raggi Random Esoteric Creature Generator, but it does the job:
- Breathes Fire (3-18 pts, cone 10 feet long at base)
- Has Shocking Grasp (1-8 electrical damage per touch)
- Poisonous breath (cloud 10x10x10, sv vs poison or die, immune to own breath)
- Spellcaster (mage or priest of 1st-5th level)
- Touch causes disease (as 3rd level AD&D spell)
- Immune to attack form (Cold, Fire, Poison, En/Charm, etc)
- Uses a magic weapon in combat
- Uses missile weapons in combat
- Reflects magic on caster
- Smarter than the average bear
- Uses magic item (ring, potion, amulet, etc)
- Unusual alliance
For unusual powers like the above, I use the "touched by the gods" explanation in my campaign. In certain creatures, powers develop that mean that one is favored by the gods (may or may not be, it may be a mutation due to any reason) and they are often at the top of the food chain (natural leaders) of their group. Sometimes, however, their unusual powers make them outcasts and they will be found by themselves in a secluded lair, nursing their hatred at the world.
Breathes fire: self explanatory, can use once a round or turn;
Shocking grasp: can either turn it off or it is an continuous effect;
Poisonous breath: again, once a round or turn;
Spellcaster: Creature is particularly intelligent or wise (15-18) for it's kind, and has access to a spellbook and training or worships a god that answers it's call;
Touch causes disease: as the cleric spell, usually a worshipper of Bacaris (the god of Disease and Filth in my campaign, he often gives this boon to his worshippers hoping to spread plague)
Has a magic weapon: Got it from a foe defeated in battle or found it in a treasure horde;
Immune to a special attack form: choose randomly or use to confound expectations ( a troll immune to fire, for example, or a fire giant immune to cold);
Reflect magic: Only for "targeted" spells like Magic Missile or can expand to area spells;
Use Missile weapons: Many creatures would benefit from being able to fire a bow or even throw a spear or two before combat;
Smarter than the average bear: unlike most of it’s kind, the creature is a natural and cunning leader, of higher wisdom/intelligence, and is able to do some abstract thinking, use sophisticated battle tactics, and create devious traps/ambushes;
Uses a magic item: gathered from a defeated foe or found in a treasure horde;
Unusual alliance: has overcome it’s natural bestial hatred or hunger for others and entered into a mutually beneficial alliance with another creature.
Examples of each that could spice up your game and confound player expectations:
A Minotaur with fire breath
A Mummy with a shocking grasp
A Naga with poisonous breath (in addition to or in lieu of its poisonous bite)
A Medusa who is a 3rd level mage with Charm Person, Magic Missile, and Invisibility
A Carrion Crawler whose tentacles cause disease instead of/in addition to paralyzing
A Wolfwere with a +2 longsword
A Troll immune to non-magical weapons
A Spectre that reflects magic
A group of Gargoyles that uses longbows before they fly to attack
A Displacer Beast who is a genius of it’s kind….a leader of the pack extraodinaire, it has turned the other beasts into a well-trained fighting force; they use tactics like ganging up on one character to kill them before moving to another, bounding past fighters to attack spellcasters, and setting ambushes in the caves where they lair (aided by howls and barks of the genius Beast)
A Lamia with a ring of fire resistance and a potion of extra healing
A pair of Dopplegangers who have formed an alliance with a Deathkiss beholder. They wait in the wilderness and take on the appearance of a merchant and his mule being attacked by the deathkiss. As soon as the party engages the deathkiss and attempts to rescue the “merchant and beast”, the pair will attack by surprise.
It goes without saying that such unusual permutaions shouldn't be overused, lest your campaign become a "random monster" session and lose a lot of credibility. If every goblin, orc and werewolf is wielding a magic sword, wand of paralyzation and firing lasers from their frikken eyes, the campaign starts to resemble a particularly jokey version of Gamma World or Mutant Future. I also like to leave some sort of small clue that "not everything is quite kosher" to train your players to be more observant. Perhaps the bodies in the minotaur's lair are burned beyond recognition, or the one survivor of the medusa's fury says "she appeared out of nowhere, I swear!" while recovering at the local inn.
Remember to bump that XP reward, while you're at it!