These are monsters that hide in the environment (walls, floors, ceilings, etc.) and never fully appear until you walk close to them, at which point they suddenly lunge out and try to grab you. Bonus points if they're also Mook Bouncers and eject you elsewhere when they grab you.
Can be extremely annoying, especially when they guard narrow ledges, as you can't target them until you're within their attacking range.
Compare Goddamned Bats and Demonic Spiders, depending on what the particular monster is capable of. See also Chest Monster for enemies that hide by disguising themselves as something benign.
Pictured to the right: a Floormaster in The Legend of Zelda: The Wind Waker. Its attack is accompanied by a terrifying cry that will likely make your heart skip a beat. Floormasters in other Zelda games are merely Mook Bouncers that move around normally.
In at least one version, there was a different color of Wall Master that would kill you rather than bounce you out.
In fact, Wall Masters have been named the video game enemy most likely to give the player a heart attack.
In the Shadow Temple and Spirit Temple, they're also invisible.
Even given an Expy / Shout-Out in the Zant's Hands in The Legend of Zelda: Twilight Princess, where they don't grab you, they slam into you from above and grab your Macguffin when you (naturally) drop it. And in case that didn't already have you on edge, their approach is accompanied by increasingly frantic Psycho Strings.
The Legend of Zelda: Spirit Tracks has its own Expy / Shout-Out in the form of Key Masters, which look like the original Wall Masters but act more like Zant's Hands. In several dungeons, a bunch of them jump out of the ground the second you pick up the Big Key, and would try to steal it from you until you finally threw it into the keyhole.
The dead hand from The Legend of Zelda: Ocarina of Time was an annoying pile of white rotting flesh that you fought at the bottom of the well and in the Shadow Temple. It hid in the ground, only exposing its numerous, gangly arms. If you got too close to one of the arms, it grabbed you and the monster itself would rise up from the ground. The creature may not be as subtle as the wall masters, but still...
The games also feature Deku Scrubs and Mad Scrubs, which hide underneath bushes until approached (or if you get too close).
Also in Ocarina, the cursed Skulltula patriarch caused more than one player to leap out of their chairs screaming. He's not hostile, but you wouldn't guess that from his appearance — and the way he drops down on you from the ceiling of a creepy house.
It's not just there, big Skulltulas drop around all over the game. Great Deku Tree, Forest Temple, and Shadow Temple are where they're most common; though they show up in Bottom of the Well also. Nothing quite like a spider the size of your clydesdale dropping into your face just over an entrance.
Also there are these "doors" that would attack you if you try to open them. Not to mention the pots and floor tiles that looked nothing out of the ordinary until you got too close and they would hover for a second and then hurled themselves at you.
The first two Metroid Prime games had wall-dwelling Reaper Vines, Aqua Reapers, and Darkling Tentacles that acted this way.
Some the Metroids in the original NES Metroid game are kind of like this, in that they lurk just off screen then rush you as soon as they appear, and a few do start off in the ceiling or floor.
Metroid: Other M has the Whipvine, which acts similarly to the various tentacles from the first two Prime games, and the Grippers, which pretend to be random flowers growing in the background before jumping out and eating your head when you walk past.
The Kyoma Demon monster in Castlevania: Aria of Sorrow hides in a few of the many mirrors in the Inner Quarters area and mimics your movements in the background when you pass by it; stand in front of the mirror it's hiding in for too long, and it pops out to swipe at you with his sword. Castlevania: Harmony of Dissonance has a similar enemy in the Sky Walkway area.
The original Wall Master in the Castlevania series is the venerable Fish Man, which will pop out of the water in sewer-based levels the moment you get too close to the edge of an overhang.
The boss monster Paranoia in the sequel Dawn of Sorrow does much the same thing, but mostly uses lasers while hiding in the mirrors.
Sandcrocs in Cave Story. Make the mistake of stepping on the sand in the wrong place, and BAM! Big chompy jaws outta nowhere that deal insane damage. You can't shoot them until they pop out, and they don't do that until you're standing in their bite range, so killing them isn't really an option either. Plus that first one you're not expecting can scare the hell out of you. There's also a possessed door which reveals itself only when standing close to it. It catches most people off guard.
God of War II and III have "Servants of Hades", which are revealed to be the hands which fully-corrupt sinful humans, turning them into the red zombie undeads commonly found in Hades, and prepare them for their transformation into the Zombie Soldier mooks that Kratos has fought IN ALMOST EVERY GAME. They're generally weak, but they act like unpassable grass when not sliced apart.
Mega Man Legends 2 has Shoebafun, Reaverbots that hide in the floor and don't come out until you're on top of them, at which point they pop out of the floor and clamp shut on your legs. The only way to avoid them is to either jump exactly when you see one coming up or button mash like hell once they've got you.
The "Bucculus" in Super Smash Bros. Brawl's Subspace Emissary. You can just see the tip of its lips poking out of the ground, but drawing it out without getting it stuck on you is a real pain.
First Person Shooter
In Star WarsDark Forces Saga, there's a level where you have to go through a sewer infested with the tentacle monsters from A New Hope. Specifically, they would just jump out of the muddy water and attack you. There are tales of players who felt like they were having a heart attack when they appeared.
Most of the special zombies in Left 4 Dead are Wall Masters. The Hunter is probably the best example of the trope; he appears out of nowhere to pin down stragglers and rip them apart. The director AI of Left 4 Dead 2 has a nasty habit of putting Chargers right behind/beside the group; as it often does this following Jerkass behavior, players have dubbed them "Karma Chargers".
Half-Life 2 had the entire city of Ravenholm, where EVERYBODY had died. Corpses laid everywhere, and you could hear the moan of zombies as fires roared. The whole place was Nightmare Fuel. But seeing as how it's Valve, they took it Up to Eleven with the fact that some zombies will disguise themselves as normal corpses only getting up after you're right next to them. Or maybe even until you had passed them... It says a lot about a game when corpses can be considered part of the scenery.
Nitemare 3D had two types of statues that came to life when you got close enough, one for the stone-walled areas and one for the hedge mazes.
The Xenomorphs in the Aliens Versus Predator videogames are very good at hiding in walls, ceilings and corners, especially in their own hives, where they will hide in plain sight until the walls and ceilings start to move.
World of WarcraftExpansion PackThe Burning Crusade introduced sand worms. Though they could be seen by bouncing rock on the ground, they couldn't actually be targeted until you enter within melee range of them, making them a headache for Glass Cannon casters. Another variation are enemies that appear when you interact with certain objects.
A variant of this appears in The Eden Trial in City of Heroes with the Rock Wall and Mold Wall. Each is a wall that must be destroyed by the players to progress through the trial, each has a lot of hitpoints, and both spawn monsters from the wall itself when it is attacked.
Runescape includes the aptly named Wall Beast. It's lair is more or less visible, but is so placed to be nearly unavoidable. If you walk past it, a giant claw reaches out and grabs you by the head, shaking you and inflicting major damage. Averted if you are wearing a spiked helmet, in which case you can just kill the thing normally.
Guild Wars has a large assortment of "pop-up" mobs that only appear when you approach their location. This includes rappelling bandits, dormant spirits, burrowed insects, burrowed undead, burrowed plants, ceiling-hanging bats, ambushing demons, and wurms. Prophecies was especially fond of this, often leaving large areas of zones suspiciously devoid of any enemies.
Grille Chompa, Portrait Chompa, Grimlet and Big Clucker in Banjo-Kazooie and Snapdragon, Dragunda, and Hothand in Banjo-Tooie.
Eversion has the GODDAMN HANDS, which emit an extremely irritating screech whenever they appear. Find them in Level 4.
I Wanna Be the Guy has a lot of enemies that come at you extremely fast from out of nowhere. An enemy more clearly fitting the Wall Master profile is Master Claw, which comes out of tree hollows in the Ghouls'n Ghosts level and reappears in the Castle of the Guy where you least expect it. A certain pit of death has Ryu pop out of nowhere spinning in a continuous Hurricane Kick.
In Episode 4, the Wormouths look like little green worm things that go back and forth over an area of the ground fairly quickly, then when they get close enough to you, they show themselves to have a surprisingly big mouth full of teeth. They can only be shot if you shoot them at just the right time as they try to attack you. Many a player has lost more than one life to these nasty little buggers.
Also in Episode 4, there's the walking rocks which only turn active when you're not facing them which fit the definition more closely. They look like typical rocks you see at the side of the road, although they can be recognized by sitting on the plane where you walk, instead of the foreground. When you're not facing them, they walk around, and when they're at the correct distance they jump to knock you out.
Episode 6 has the Ceilick, which hides in a hole in the ceiling and tries to poke you with its tongue, then laughs at its antics. Can be a problem if they're near a key gem. Still, at least you can shoot them when they're laughing.
In Klonoa 2: Lunatea's Veil'', there is a plant-like monster that lurks within tunnels in the walls and will try to eat Klonoa if he gets too close. A Moo needs to be thrown in its way as bait in order to get past.
Inverted by an enemy from Sonic The Hedgehog 3's Marble Garden Zone. They look like spikes until you get close enough to them, but the projectiles they shoot are easy to dodge (and can be repelled by any shield)—and their spikes act like springs rather than hurting you. In other words, they're less dangerous than the landscape feature they're imitating. Played straight in Sonic the Hedgehog 1 with the Burrobot, and in Sonic the Hedgehog 2 with its spiritual successor, Grounder.
Monster Bash has hands with eyes hiding in walls which throw knives and which simply cannot be killed. On the bright side, other monsters are not immune to the knives they throw.
Crystal Caves has the purple snake-dinosaur thing which also makes a cameo at the title screen. It's usually just a small purple bump in the wall, but if you get too close, it emerges, snaps its jaws once and hides back.
The last stage of the CapcomArcade GameNemo has the Nightmare King's enormous clawed hands emerging from numerous holes in the walls.
Holy Diver has a enemy that pops up from the middle of small pillars and bounces up and down.
The mutant rats from the almost unknown but hilariously bad game Revolution.
Real Time Strategy
Various guns, flamethrowers, and missile launchers appear out of the floor and walls during the indoor missions in StarCraft. There's also the Zerg ground forces. Most of them can bury themselves to prepare an ambush, and the Lurker can even attack from underground. StarCraft II also introduces Nydus Worms which can dig through the ground in order to serve as entrances to an underground network for any Zerg forces.
Can be done in the Total War games (Rome, Medieval II, Empire) where an army that stands close enough to a thickly forested area can hide, and ambush any other army that comes its way. Makes it hellish when your city is being besieged by a lackluster force, and you send a full stack to rescue it, only to be AMBUSHED by an EVEN BIGGER ARMY on the way there. To make it fair, the player can set up armies to ambush, too!
The Monster Houses in Pokémon Mystery Dungeon (both generations) are a kind of Wall Master, but plural. An empty room with lots of items and Wonder Tiles may spawn a horde of enemy Pokemon when you enter, and unless you are either immensely overleveled, can find a way to retreat into a corridor (on some occasions, without exposing your partners) and beat down the Pokemon one by one, or have the moves and Orbs necessary to immobilize the bunch, it's almost a guaranteed loss. Made even worse in the sequel when you can't use orbs against them.
Ghost Pokemon can come at you through walls. You can see them when they do this, but most attacks will fail. You have to run a few steps to trick them into coming out where you can hurt them. And meanwhile they knock out the weak guy you were leveling up...
Many Roguelikes have mimic monsters that pretend to be items or other objects before the player comes close. Dungeon Crawl has a particularly varied bunch: mimics can pretend to be anything from potions and scrolls to doors, stairways and even Bonus Dungeon entrances. Also, mimics pretending to be the entrance to a branch will get all the special features which would normally surround the entrance. The last thing you'd expect is that a bunch of giant spiders and spider webs are just guarding a mimic pretending to be the entrance to the spider's nest.
Role Playing Games
Evil Murals in Blue Dragon. Forgivable because we had fair warning and they're somewhat noticeable. Plus you get a treasure chest after beating them, which is nice.
Those undead maneating Venus flytrap things in The Witcher.
Chigoe bugs in Final Fantasy XI are almost impossible to visually distinguish from the dirt and grass underneath them, and don't have their names appear above their heads like most monsters until they're attacking you. When they attack you, they attack at insanely high speeds and can cause the Disease status on each hit. They also give almost nothing for defeating them.
The Wallbeast in Shining in the Darkness' and Shining The Holy Ark''. The name says it all. Bonus deception because of the fact that the walls you had to open had an eye design on them, just like a creature disguised as a wall. In fact, you still had to open these; it just led to a battle.
Floors and grass in Ultima III, both of which just look like ordinary tiles; ghosts and Shadowlords can come at you through walls and be invisible in Ultima V - and do both at the same time.
A number of the unique monsters in Xenoblade Chronicles won't show themselves until you walk extremely close to where they're hiding, at which point they'll leap out of the water, drop from the ceiling or sky, or climb up the edge of a cliff.
The wall hand things in Ib. Beware the edges indeed!
In MOTHER 3 there are Pseudoors, which look just like normal doors but will attack you if you try to open them.
The Wall Snatchers from Silent Hill 4: The Room. Imagine a monster that sprouts suddenly out of the walls when the protagonist gets close. Imagine that it does a substantial amount of damage when it hits you. Imagine that every hit knocks you back a fair distance. Now imagine that there are hundreds of them hiding in the walls of a very long, very narrow escalator. The Victims also have the ability to materialize out of walls.
Minecraft has Silverfish, which start out as a special type of stone block that mines curiously fast if you don't have a pick, and if attacked will summon their brethren to come out of hiding automatically. Once they start spawning, the best strategy is to just get the hell out of Dodge and wait for them to go into the surrounding blocks, which when mined will start the whole thing over again. With some updates, players have the ability to place these types of wall masters in creative mode wherever they please, which is most likely for people creating custom maps with traps. In survival mode, if you mine these special blocks with the silk touch ability in your pickaxe, you can harvest the booby trapped block and place them anywhere else as traps for your friends. The item is even called "Monster Egg" in the inventory.
Non-Video Game Examples:
Anime and Manga
The Paradox Brothers in Yu-Gi-Oh! used a card called "Wall Shadow" which could cheat the maze setup of the duel and move along the walls to get anywhere. The card exists in the actual TCG, too, but its effect merely makes it unsummonable without tributing a "Labyrinth Wall" with a "Magical Labyrinth" equipped to it. Aside from that, Wall Shadow has very good defense and mediocre attacking ability... which isn't very helpful, since its defense is equal to the "Labyrinth Wall"'s and the attack of the former card is nonexistent, so you wouldn't be using it for offense anyways.
Suprise zombies in One Piece were capable of disguising themselves as furniture, and lived up to their name at first. However, most of them were utterly incompetent at anything else. The Negative Hollows Perona used later in the same arc also had a tendency to drift ponderously out of walls.
The eponymous Alien excells at clinging at walls and ceilings to ambush and grab a soon to be host. Notable in the second movie, where the creatures are introduced five minutes before we realize so, when what we though was a wall turned out to be more dangerous. The last scene of the original movie pulls a similar trick.
The "cavernmouths" from The Seventh Tower. They look like caves... until you go too close and CHOMP! enormous jaws shoot out at lightning speed and eat you. Occasionally can be distinguished by a pair of Glowing Eyes of Doom in the depths of the cave.
A rare (but oft-remembered) non-video game version of this is present in the Nickelodeon gameshow Legends of the Hidden Temple in the form of Temple Guards, who would forcibly grab players if they ventured into the wrong room or checked the wrong part of a room, taking away a "pendant of life" or removing the player from the game if they did not have one.
Incorporeal creatures in Dungeons & Dragons can lurk inside objects and listen for enemies (especially the player characters) to approach. (This is in addition to all the enemies that impersonate walls, ceilings, floors, and furniture.) Oddly enough, this tactic doesn't seem to be used in D&D video games, even when incorporeal enemies appear.
Another Dungeons & Dragons example is the "room of death" which has shown up in several adventures. The Carpet, Rug, and Wall Drapings are all animated objects and try to strangle you. The desk and chair, as well as the chest in the corner, are all Mimics, and once you're distracted spring forward and attack you. The monsters are all visible, but only a total paranoid or someone with a lot of experience with D&D would expect them to actually be enemies.