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. A similar trope for enemies capable of speech is Simultaneous Warning And Action. If you thought "beating" meant "defeat", you may be looking for Pre-Mortem One-Liner.
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Anime and Manga
The Rogue Titan from Attack on Titan starts off every major battle (usually involving tearing every other Titan in its vicinity a new asshole) with a terrifying roar. Justified in that Eren is usually in the throes of Unstoppable Rage whenever he's in this form.
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.
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."
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).
The X-Files: Occurs in some Monster Of The Week episodes, e.g. 1x02 Squeeze, 6x16 Alpha, 8x04 Patience.
Most of the Skag types in Borderlands open up their three jaws to screech at you before charging...which is incrediblystupid. 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.
In Borderlands 2, when triggering Release the Beast, Krieg may let out a monstrous roar as he mutates into a Badass Psycho. Luckily, he's invulnerable during the transformation.
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.
This is a good way to know how badly Kratos wants you dead.
Half-Life: The headcrabs who emit a screech when throwing themselves on the player. Especially notorious are the black headcrabs from Half-Life 2 who screech even before attacking their prey. Zombies might or might not qualify, dependent on how much their vocal function is controlled by the headcrab.
The Half-Life 2 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 samereasons that Fast Zombies stop and scream.
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 Warcraft III's teaser trailer an Infernal falls from the sky, crawls from its flaming crater, and roars at the footman and grunt facing it.
In-game, any unit with Roar (increases ally damage) or Howl of Terror (decreases enemy damage) will do this before attacking.
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 (Or when they get ticked off), but it's not always 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 (Unless you have the Earplugs ability). A few monsters like the Tigrex and Akantor even roar loudly enough to injure you and send you flying if you're too close.
In Dragon Age: Origins, the Grey Warden is tasked with reaching the top of the darkspawn-infested Tower of Ishal to light a beacon. An ogre is hunched over in front of the beacon, gnawing on something. When it notices you, it lets out a massive roar before attacking.
Another ogre does the same thing to King Cailan before killing him.
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.
A wyvern also lets out a roar before attacking you in the Mark of the Assassin DLC.
In Super Meat Boy, fourth and fifth boss roar ingame and cutscene (and the third roars only in cutscene). Almost parodied with the fifth boss, whose cutscene has it roaring in three-part harmony, and the third, who roars at you just before having the fight interrupted (and never actually gets around to fighting you).
All the bosses in Patapon roar if you attack them while they're resting, or get close enough
In Ōkami, Orochi lets out a particularly badass roar whenever he makes a major appearance.
Endermen from Minecraft make a rather disturbing noise (at 0:35 in this video) should the player provoke them by "staring" at them (moving the crosshairs directly over their torsos or heads). Then they usually teleport directly behind the player.
Off-White: The wolves are shown growling at a elk before killing it.
Animals such as grizzly bears might seem to do it as survivor accounts sometimes relate, but, as stated above what's 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.
Same with rattlesnakes. They're infamous for their warnings before attacking, unless they're hunting for food, when they stay quiet.
Soldiers would often roar while charging with spears and later bayonets. May originate with the behavior of certain animals among themselves in territorial disputes, or fighting over a mate. Especially in premodern conflicts, intimidating battle cries were great for making the enemy break ranks and run to prevent a full on fight. Sun Tzu advised generals to leave the enemy an avenue of escape because an enemy fighting to the death is far more dangerous than an enemy running away.
The general idea is that animals make noise when they want to intimidate, they shut up when they want to eat.