Gotcha Monster is a slang term used in /tg/ circles to refer to any of the wide array of monsters whose hunting strategy revolves around disguising themselves or lurking in seemingly innocuous places and then trying to kill adventurers the moment that they let their guard down even the slightest. The term is most associated with Advanced Dungeons & Dragons and Old School Roleplaying, which gave rise to some of the most infamous species of Gotcha Monsters in existence, but the basic idea can be found pretty much throughout gaming circles.
D&D Gotcha Monsters
Animated Object: Could be a table, a chair, or just about anything in the room waiting to surprise you.
Brain Mole: An ordinary mole that happens to feed on mental energy, resulting in it draining away memories and potentially leaving its victims as drooling vegetables if they get too close to its den.
Cloaker: A killer manta ray that disguises itself as a cloak to trick people into wearing it, which lets it eat them.
Cushion Fungi: Carnivorous fungi that look like pillows so people will sleep on them.
Shrieker: Mushrooms that scream really loudly and attract other monsters to you if you disturb them.
Gargoyle: A vicious predatory creature that disguises itself as a statue in order to attack prey when their guard is down.
Ear Seeker: A wood-eating insect that often infests dungeons doors, and will bore into the ears of anyone who tries to listen at those doors, because they like to lay their eggs in the ear canals of humanoids so their larvae can feed on living brain tissue.
Throat Leech: A small leech that infests ponds and streams, and which prefers to feed by swimming down the throat of anyone drinking the water so it can attach itself to the inside of the throat and suck their blood, incidentally suffocating them as it swells up.
Living Wall: A mass of undead flesh and bone melded together, which uses innate illusions to disguise itself as an ordinary stone wall until something living comes close, whereupon it tries to rip them apart and absorb them into its mass.
Lurker: A flattish monster, somewhere between a slime and a flying manta ray, that clings to the ceiling and then drops on people to smother them.
Trapper: A bigger version of the Lurker, which disguises itself as a dungeon or cavern floor and then rises up to crush and devour anyone who steps on it.
Miner: A forest-dwelling variant of the Trapper, which hides itself on forest trails and waits until victims step on its back, stabbing themselves on its numerous paralytic poison-carrying quills so it can then smother and devour them.
Giant Black Squirrel: A 2ft-long, black-furred relative of the squirrel that inhabits "evil-dominated forests". Harmless, but likes to steal valuables from travelers.
Piercer: A carnivorous gastropod which resembles a stalagmite; it clings to cavern ceilings and then attempts to drop on anything that moves below, hoping to fatally impale them on its sharpened shell
Choke Creeper: A predatory vine-like plant that drapes itself across tree branches, and attempts to strangle any animal that gets within range.
Roper: A tentacled monster that disguises itself as a rock outcropping, boulder or stalagmite until victims get within range.
Rust Monster: A giant beetle-like creature that feeds on metal, dissolving arms and armor in pursuit of food.
Heway: An intelligent snake which hunts by climbing into wells or ponds and then shedding its skin, which transmutes the water into a toxic fluid; it then eats anything killed by drinking the fouled water.
Treant: An intelligent tree which will rip you to pieces if it thinks you're destroying the forest. Fortunately, they're technically good guys, so you won't usually need to fight them.
Rot Grub: Voracious predatory maggots which infest mounds of dung and carrion, and which will leap to infest and devour living creatures that get too close.
Adherer: A monstrous humanoid which looks like a mummy, but which is actually a living being covered in fibrous skin-strands that secrete a gluey resin. They use these to catch an adventurer's weapon and rip it out of his grasp, whereupon they can beat him to death.
Disenchanter: A creature resembling a blue camel with an elephant's trunk that sucks the magical energy out of magic items.
Gambado: A predatory monster that hides itself in a pit with just its head - covered in a seemingly abandoned skull - poking out. It then jumps out and tries to kill anyone who gets too close.
Sheet Phantom: A malevolent undead which possesses a thick blanket and uses it as a physical body to try and grapple, trap and smother living beings.
Stunjelly: A variant of the Gelatinous Cube which clings to a single section of stone wall and paralyzes anyone who touches it. Then it oozes over their body and eats them.
Yellow Musk Creeper: A pretty creeping vine with beautiful yellow flowers. It lures victims in with its sweet scent, and then plants seeds in their heads, which consume the host's brain and then hijack the body as a zombie they pilot to find a new place to grow.
Aballin: A slime-like creature that resembles a mass of animate water, which it pretends to be before drowning and digesting anything that attempts to drink from it.
Carrionette: A sapient, malicious marionette-type puppet that possesses independent motion and a malicious desire to kill people.
Lock Lurker: A vicious invertebrate that resembles a gold coin, until it extends its legs and the lethally poisonous, scorpion-like stinging tail it normally hides in the Ethereal Plane.
Noran: A malevolent version of the Treant which lurks in underground environments.
Bloodring: A blood-drinking parasitic worm that disguises itself by pretending to be an enchanted ring, as it can extend its own innate magic resistance to any creature it is touching.
Goldbug: A blood-sucking beetle that disguises itself as a gold coin to get close to its humanoid prey.
Wizard Lice: Blood-sucking insects that would be merely annoying if they didn't strip the spells right out of the minds of spellcaster hosts.
Vampire Moss: A mass of hanging, dense moss which passively drains the life force from any living creature that gets within 10 yards.
Heart Tick: An oversized tick that latches onto victims with its barbed legs so it can puncture their heart and siphon out the richest, most potent blood in their body.
Dark Tree: A malevolent counterpart to the Treant.
Singing Tree: Another malevolent counterpart to the Treant, which uses hypnotically alluring songs to bring victims closer.
Xaver: A ferrovorous creature which disguises itself as a sword in order to get closer to unprotected metal.
Living Web: A slime-like entity that disguises itself as a mass of abandoned cobwebs, then engulfs and electrocutes anything that touches it so it can consume them.
Wolf-in-Sheep's-Clothing: It doesn't look like a Sheep, and it isn't a wolf either. It looks like a small rabbit like creature, but that is only a small lure on top of its body, which looks more a tree stump.
Umbral Blot (Black ball): A construct with the appearance and properties of a Sphere of Annihilation. This might not be the best disguise since a Sphere of Annihilation isn't something you should be messing with anyway.
Genius Loci: A living, malevolent piece of landscape.
Boneleaf: An aberration that resembles a plant. It is intelligent and can use illusions lure prey. All boneleaves in an area are actually parts of a single organism so what one learns, all boneleaves nearby also learn.
Eye of the Deep: A relative of Beholders that like to fool people at sea with illusions of things like mermaids.
Creed: CREEEEEEEEEEEEEED!!! Wait no, that's 40k.
Gazebo: Beware the dread gazebo.