Roar Before Beating

Surprise--a monster! But you have time to recover from the shock of its appearance because it won't attack until it has posed dramatically and let out a Mighty Roar. Roaring is like a monster's hello. They must become hostile when heroes don't have the decency to roar a proper return greeting.

Often used as a cliffhanger to confrontations because ending after a punch is thrown seems an interruption, while a warcry is the last stop to get off at before the fight starts.

In real life, an animal that roars is warning you away. It doesn't want to fight you; it wants you to leave. An animal that tries to sneak up on you is much more likely to harm you.

Contrast Hiss Before Fleeing. Compare Transformation Name Announcement, Screaming Warrior and In the Name of The Moon, which are often the tropes you get when the hero does this. If you thought 'Beating' meant 'defeat', you may be looking for Pre Mortem One Liner.

Comic Books

 * HULK! SMASH!

Film

 * Jurassic Park does this pretty constantly.
 * Parodied in Night At the Museum, with the T. rex skeleton.
 * The cave troll in Lord of the Rings: The Fellowship of the Ring.
 * It had just smashed through a rotten old wooden door. Poor thing must have been feeling the splinters.
 * Happens twice in a row in Star Trek.
 * All Predator movies have the title alien doing this after unmasking.
 * The graboids in Tremors do this, which is especially odd because not only does it give the protagonists time to get away as per the trope, but it also totally negates their stealth advantage from being underground, and it partially blinds them--since they "see" through vibrations in the ground, sticking half their mass into the air should only make it harder to find their prey. More than one protagonist has survived an encounter simply because the creature saw fit to jump out of the ground right next to them and roar first, rather than just sneaking up under them and devouring them without warning, which is what happens to anyone who isn't a main character.
 * Justified with the shriekers, since their scream is how they navigate and identify prey.

Literature

 * Subverted in the Conan the Barbarian story "The Tower of the Elephant," which has Conan fighting off one of a pack of lions in the title tower that are completely silent when they go for the kill. He asks his partner Taurus of Nemedia why the lion he fought did not roar, and Taurus's reply was that "All things are strange in this garden. The lions strike silently -- and so do other deaths."

Live Action TV

 * The Incredible Hulk: Upon transforming, The Hulk usually gives a big roar to let the now terrified villains know that he is really pissed off.
 * Most of the enemies in Tomica Hero Rescue Force do this, as well as two of the friendly Mecha (Dozer and Shovel). Its sequel, Rescue Fire, kicks it into overdrive by having most of the friendly mecha do it as well.
 * Legendarily, the Daleks in Doctor Who always waste time shouting "EXTERMINATE!" before opening fire, giving the Doctor time to escape. Lampshaded in "The Parting of the Ways", where a Dalek gets in a firefight with a robot that's designed to spout a catchphrase before firing its disintegrator ray (the Dalek wins because, unlike the android, it can just fire without shouting first if it really wants to).

Video Games

 * Most of the Skag types in Borderlands open up their three jaws to screech at you before charging...which is incredibly stupid. Some of the most dangerous boss Skags can be shot dead before they even start fighting you thanks to this trope and an emptied magazine.
 * All bosses in the Dark Cloud series pose as the battle begins. They also get Boss Subtitles, even if the characters had been conversing with them several minutes beforehand.
 * The karkians in Deus Ex roar before charging you. The baby karkians don't, most likely because they can't kill you with one bite.
 * Demons in Doom 3 will roar at you, take a charge, then roar again every few feet.
 * The feral ghouls from Fallout 3. Unfortunately for them, this is no free action; they still take normal damage while roaring instead of charging right away, with often fatal results.
 * The large trigens from Far Cry.
 * The Final Fantasy series is fond of this before a boss fight; Bahamut is most notable.
 * Wretches in Gears of War.
 * Nine times out of ten, mooks in God Hand will stop to throw off a one-liner before charging in.
 * Euryale in God of War 2.
 * In Half-Life 2, the Fast Zombies stop and scream before they attack. Apparently this is a way of keeping them scary without making them too easy or too annoying.
 * In Left 4 Dead and the sequel, the Tank will roar and pound its chest after incapacitating a survivor, for much of the same reasons that Fast Zombies stop and scream.
 * Thresher Maws in Mass Effect.
 * Used extensively in Resident Evil to compensate for the fact that you can't move while aiming your weapon.
 * The boss at the end of the Shank trailer. Though he isn't a monster he's a monstrously large man.
 * Ugh Zan III in Serious Sam roars quite a while before he starts attacking.
 * Wyverns and Dragons in Vagrant Story roar impressively as they attack; D'Tok, the first such enemy in the game, is a notable example. Also, a variety of bosses will even cause a small pause in the action and have their own mini-cutscene instants before they initiate their ultimate attack (including the Final Boss,) relinquishing control to the player just in time to defend or flee.
 * In War Craft 3's teaser trailer an Infernal falls from the sky, crawls from its flaming crater, and roars at the footman and grunt facing it.
 * In Starcraft II cinematics, the hydralisks roar before attacking General Warfield (and subsequently one of them get punched in the face).
 * The Lord of Change at the end of the Warhammer Online Cinematic.
 * All Pokémon will roar when they enter a battle. Wild, trained, or legendary, stupid or super-genius, it doesn't matter. They'll do it.
 * Used to terrifying effect in the dinosaur hunting game Carnivores. While all the other dangerous dinosaurs just run in and kill you without making much noise until it's too late, the T-rex has poor eyesight and must sniff the air to know if you're nearby. If one smells you, it immediately lets out a horrifyingly loud roar and runs right for you. Seeing as their only weak spot is the eye (shoot anywhere else and you'll just piss them off), unless you somehow manage to shoot their eye as they're charging you, if you hear that roar you're pretty much screwed.
 * A lot of the creatures in Monster Hunter roar when they see you, but usually it's not the kind of roar that leaves you stuck covering your ears. The mook and boss raptors (-Preys and -Dromes) take a noticeable amount of time roaring, so a quick-witted hunter can easily get the first hit on them. On the other hand, wyverns and any other large boss-sized monster can deafen you with roars, stunning you briefly.
 * In Dragon Age, the Grey Warden is tasked with reaching the top of the darkspawn-infested Tower of Ishal to light a beacon. A massive Ogre is hunched over in front of the beacon, gnawing on something. When it notices you, it lets out a massive roar before attacking.
 * An ogre does the same thing to
 * In Dragon Age II, Flemeth roars at the horde of darkspawn before slaughtering them, thus saving the lives of Hawke and his companions.
 * At the climax of the Bone Pit quest, a dragon comes flying out of the sky and roars at Hawke before attacking.
 * Many Metroid bosses.
 * In Super Meat Boy, fourth and fifth boss roar ingame and cutscene. Almost parodied with the fifth boss.
 * All the bosses in Patapon roar if you attack them while they're resting, or get close enough
 * In Okami, Orochi lets out a particularly badass roar whenever he makes a major appearance.
 * The Abominable Snowbug from Bug! does this as soon as it breaks free from the ice it was trapped in.
 * The Fleshpound in Killing Floor stops for a moment to roar when it Turns Red.
 * The monsters in the Dead Space series tend to helpfully roar to inform Isaac they're about to attack, or have snuck up behind him.

Real Life

 * Animals such as grizzly bears might seem to do it as survivor accounts sometimes relate, but, as stated above what's really likely occurring is that the animal is giving a warning signal and the human(s) are unable or don't know to retreat to its liking.
 * Except rattlesnakes. They're infamous for their warnings before attacking.
 * Not when they are hunting, the rattle is a warning for whatever is bothering the snake to go away.
 * Soldiers would often roar while charging with spears and later bayonets. Probably the origin of this trope, as, as has been stated earlier, animals don't really do it.
 * Except a fair number do, within species, when in territorial disputes, or fighting over a mate. Kind of like soldiers, really...
 * The general idea is that animals make noise when they want to intimidate, they shut up when they want to eat.