The precedent was set for this from the original little booklets. From the introduction of monsters that can petrify by touch, or within 6", to Jellies, Molds, Slimes and Puddings, trick monsters have been a fundamental part of Dungeons and Dragons. They either create a dungeon hazard or something that adds a dynamic feature to a battlefield. I was in middle school the last time someone left green slime alone instead of trying to collect it to throw or befuddle some monster into walking into it.
Gary doubles down on the idea of the monster trick, in his archetypical trap list at the back of Volume 1, Greyhawk.
A small selection of that list is reproduced here:
Animals which appear to be perfectly harmless but are deadly:So from the very beginnings of the game, there were monsters which were monsters—you could hit them with swords, or talk with them or bargain with them. And there were monsters that were tricks. Creatures that had a specific vulnerability removed as given above. Mimics, molds, dopplegangers, and more. It didn't just stop at the official materials. Many "unofficial" monsters were designed as trick monsters, such as the slinger. It's not just something you fight, it's a puzzle to solve. Entire rooms or sections of hallway might be devoted to a creature, truly making it a trick encounter. Frequently "Boss" encounters in modern adventure paths often contain similar amounts of creativity: A white dragon in their lair isn't just a straightforward fight.
Oxen which are cross-bred with Gorgons. small lizards which are able to breath fire, creatures which grow to huge size if approached too closely, or animals which turn to some horrid monster if touched are typical examples.
A giant with faces or multiple heads which can never be surprised, and with four additional eyes is able to see invisible and hidden objects and co-ordinate no less than two attacks per melee round.
Giants known as "Rock Giants" which so closely resemble stone that they can be detected seldom (1 in 12 is a good percentage).
Fire-resistant mummies. Many players will get used to frying these monsters with oil. but watch the fun when they run into one of these critters!
Skeletons who are able to hurl their finger joints as if they were magic arrows.
Monsters which are in endless supply due to a magical point of origin. "Greyhawk" had a fountain on its second level which issued endless numbers of snakes.
Containers which are filled with a gas or liquid which turns into a monster if the gas or liquid is dispensed.
. . .
Of a similar nature are monsters which appear to be something other than they actually
are such as:
An Ogre Jelly monster which appears to be a mere Ogre, but. . .
A Snake which is actually Grey Oooze.
A Giant Spider-like Black Pudding.
A Symbiotic Dragon which spits Ochre Jelly, Black Pudding, etc.
A seeming Golden Dragon which is actually mobile Yellow mold.
It seems likely from what we can tell that monsters rolled for encounters were likely monsters of various factions and any standard listed encounter was probably with a trick monster. The spacious nature of the dungeon turning it the game into one long strategic encounter.
That said, here's a broad overview of some of the many types of trick monsters you could have:
- Weapons don't work. Sometimes it requires magical weapons, but some monsters may be immune to attack completely
- Spell immunity or affected only by certain specific spells (such as a golem)
- One type of monster that looks like another (gas spore, the other examples above)
- A monster with a specific weakness that for some reason doesn't have that weakness (fire-resistant mummies as above, or a half-dragon troll)
- Monsters which have only a certain critical weak points they can be hit (with varying armor classes).
- Monsters that are working in tandem and have developed specific tactics by either working with each other or the enviornment
- Monsters that look like items or other harmless things. (This category is huge: mimics, lurkers, cloakers, etc.)
- Monsters that don't do damage, but otherwise inhibit or affect the party (e.g. rust monsters, disenchanters, aurumvoraxes that eat gold). These are especially motivating if they take things from the players and flee. This doesn't necessarily have to be about things either. Monster could cause the party to rage or become insane, et. al.
- Monsters that punish players for engaging in traditional combat (petrification and level drain monsters)
- Monsters that debilitate players or remove or negate some of their effectiveness when engaged (Anti-magic cone of a beholder, confusion gaze of an umber hulk)
- Monsters, that by their nature, appear non-monstrous (e.g. items floating in a hallway are actually a gelatinous cube, crawling claws and undead both seem like non-living dead bodies).
- Monsters that have non-standard abilities: Spellcasting, mutations, breath weapons, etc.
- Monsters which cannot be killed (more akin to hazards) or stopped (such as generators) or monsters that can only be killed by affecting their environment (a lich's phylactery for example)
- Unkillable monsters that come back to life once slain, or once killed, metamorphize into a new form
- "Monsters" that are features or traps that can be killed (disabled), i.e. living wall
- Monsters that are parasites (rot grubs, assassin flies)
- Monsters that use magical items or other equipment
Further sources that contain more specific examples include the Tome of Adventure Design and the 5th edition Dungeon Masters Guide, both of which contain many examples of ways to modify and use monsters as tricks. A perusal of this list may note that this seems to be an entire category of creatures all to itself, perhaps truly deserving of the name "monster".
Hack & Slash