Mighty Roar

Go Lions, Brisbane Lions, we'll kick the winning score!

You will hear our mighty roar!

—Brisbane Lions Club Song (to the tune of La Marseillaise)

It's quite simple: Big monsters have big lungs, that let them make big noises. Sometimes little monsters make big noises, too, but that's neither here nor there. When a T. rex, lion, tiger, dragon or some other scary monster appears in a work that contains sound, it will make a mighty, terrifying, awe-inspiring ROOOOAAAAAARRR!!!!

If the creature roars before attacking someone, it's Roar Before Beating.

Examples of Mighty Roar include:

Anime

The nine-tailed Kyuubi from Naruto deserves a mention with a roar powerful enough to cause miniature destruction across its path.
Pokémon:
- Generally, if a Pokémon is not repeating it's name over and over again it has one of these.
  - Special mention to Palkia, Dialga, and Giratina for having kaiju roars out of the Godzilla movies!

Godzilla has a unique roar that all his fans recognize.
- Not just him, either.
As mentioned with the T. rex reference in the description, dinosaurs have a tendency to do this. Jurassic Park comes to mind as a specific example.
- This particular roar has been used and parodied in other mediums, especially by Rex in Toy Story 3 during their kid's imaginative adventures.
King Kong isn't the quietest gorilla in the jungle, either, for that matter.
The Metro Goldwyn Mayer film studio has a roaring lion at the start of each of its movies.
Little baby Clover from Cloverfield may have the most impressive set of lungs out of all the monsters. In fact one of the OST songs is explicitly called "Roar".
The shark from Jaws the Revenge actually roars like a lion in one of the most egregious cases of Artistic Licence Biology.
In the movie Primeval, Very Loosely Based on a True Story of a 20-ton giant man-eating crocodile in Africa, the giant reptile lets out a lion-like roar after devouring the Big Bad.

The Chronicles of Narnia: Lion Jesus Aslan has one of these.
Temeraire combines this with Make Me Wanna Shout.
Tarzan has his signature roar, described in the book as the victory cry of a bull ape. Unique example in that the character is a human, though he was raised as an animal.

Rover on The Prisoner has a distinct and decidedly creepy roar (perhaps made creepier by the fact that Rover is a sentient weather balloon).
Several Power Rangers animal zords roar. First and foremost are the Wild Zords of Power Rangers Wild Force; the main Megazord's regular Finishing Move is even the component zords roaring Frickin' Laser Beams.

In Star Fox Adventures, the King Redeye puts his head into the room to roar before fully entering. On an earlier note, the Galdon does some roaring, as well.
Pokémon has the move Roar, which makes the opponent Pokemon switch or flee the battle. Of course, it's learned by a lot of big Pokemon, but is specially notable on the legendary beasts of the second generation, which use it to escape from you if you ever find them and block their exit.
- Bizarrely, the Vulpix line, Lillipup line, Growlithe line, Snubbull line, Houndour line and Tepig line learn this move. Apparently in the Pokemon world dogs and pigs roar.
Okami's very own Orochi has one, and it's the key to defeating him. When he roars, your pour Sake into his mouth when he roars to lower his barrier.
- Gekigami has one of these. Fitting, as he's a tiger god of thunderstorms.
From Sengoku Basara, the Tiger's most powerful attack in an awesome howl that knocks you back several feet and makes the ground tremble.
In Spore, the special ability in the creature stage that is rewarded upon completing cell stage as a carnivore is Raging Roar, which makes all nearby creatures run away in fear.
While asura in Asura's Wrath usually does a lot of screaming, His transformation into Berserker asura destroys tons of ships just from it's shockwave. When they are above the earths Orbit. It's his default scream in Berserker form as well.

The Lion King has these right on cue for each Crowning Moment of Awesome.
- Though when Simba's still a cub, this ends up being an anticlimax. His "roar" is a kitten-like "Reeer!". Truth in Television, as seen here.
- When the hyenas were chasing Simba and Nala, Simba attempted to chase them off with a small roar, which the hyenas laugh at. He tries it again, this time with a startling unexpected surprise.
- And of course the Awesome Moment of Crowning at the end and boy is it ever.
James P. Sullivan on Monsters, Inc.. Comically averted by his very tall friend Ted, who clucks. (They wanted the Godzilla roar, but couldn't get clearance.)
- In the credits, there's a blooper reel, in which Rex auditions. He does roar. In his own...special way.
Gargoyles let loose a Mighty Roar when they wake in the evening. They can also make some pretty bestial sounds depending on how ticked off they are.
Leobreaker and Scourge of Transformers Cybertron can let out some pretty impressive roars.
A Crowning Moment of Awesome moment happens in the Tom and Jerry episode "The Milk Waif" where Tom sets off Jerry's Berserk Button when witnessing him slapping baby orphan mouse Nibbles with a flyswatter. In a rage he lets out a ferocious roar before pummeling the cat into submission.
The shark-men heroes from Street Sharks bellow roars.

The lion has the loudest roar of all the big cats which can be heard up to 5 miles away.
- The other big cats aren't so bad either.