Canis Major

A giant canine. Any canid will do: domestic dogs, wolves, coyotes, foxes, etc., as long as it's far bigger than any in real life.

A Sub Trope of Animals Not to Scale.

A Sister Trope to Mega Neko, Rodents of Unusual Size.

Compare Big Friendly Dog, Precious Puppies, Hell Hound, Cute Giant.

Compare Big Badass Wolf, which is when this trope gets scary.

Anime and Manga

 * Fenrir's real form in Mythical Detective Loki Ragnarok; see below under Norse Mythology.
 * Digimon Tamers - Dobermon, a giant doberman pinscher digimon, accompanied by a human partner, is sent by the Digimon Sovereigns to assist the tamers in battling the D-reaper by giving them the ability to biomerge in the real world.
 * Gabumon's evolved form, Garurumon, should also count.
 * There's also the shiba-inu dog, Gaogamon, in Digimon Data Squad.
 * Seasarmon, the lion-dog digimon, and Caturamon the Dog-deva digimon.
 * Cerberumon.
 * Spice and Wolf - Holo is a harvest diety whose true form is a very, VERY large wolf.
 * One Piece featured a zombie Cerberus, and a squad of dog-riders in Enies Lobby. There was also wolf-man Jabura, and possibly Mr. 4's dog-gun Lassoo.
 * Naruto
 * Akamaru grows quite huge after the Shippuden timeskip.
 * The nine-tailed fox, who was quite large when he was going around rampaging. It's interesting to wonder how large he actually is now that he's sealed inside Naruto. The one-tailed tanuki too because although tanukis resemble raccoons greatly, they're actually dogs.
 * Bleach: Captain furry Komamura is the largest captain. He's also a Big Badass Wolf even though characters in-universe think he's a dog.
 * In Inuyasha, Sesshomaru, his father and his mother all have true forms that are this.
 * Moro from Princess Mononoke.
 * Her (non-San) children are quite a bit larger than normal wolves, and are implied to be young children.
 * She is indeed considerably bigger compared to them.
 * The second Monster of the Week in Magical Girl Lyrical Nanoha. Unlike the later Mega Neko, this erred more towards the monstrous-looking side.
 * Kotarou's full beast form in Mahou Sensei Negima. The series also had a Cerberus, complete with the mane of snakes and dragon tail of the original Greek versions that most modern depictions omit. It didn't have the right number of heads, so Nodoka also theorizes that it might actually be Orthrus.
 * Duklyon: Clamp School Defenders - Ineffectual villain Imonoyama uses a growth ray to enlarge the duo's beloved dog, Inuko, into a giant monster dog. Fortunately, she still retains her playfull personality and is "defeated" when an equally enlarged Takeshi commands her to shake hands with him, awwwww!
 * Pluto from Black Butler, which can turn into a White Haired Pretty Boy, but keeps a dog mind. And can breath fire.
 * Kaibutsu from Ginga Densetsu Weed. According to the Ginga wikia, he weighs at around 1200 pounds.

Comics

 * John Constantine of Hellblazer once had to put down a ghost-possessed bulldog the size of a rhino.

Film

 * King Cesar/Seesar/Shisha/Shiisa from the Godzilla movies. Maybe.
 * Digby, the Biggest Dog in the World

Literature
"Was there ever another dog the size of a small horse, with armor plates on his sides, and jaws like a trap for bears?"
 * Pictured above: Clifford the Big Red Dog. The girl over there is his owner, Emily Elizabeth.
 * The one-and-only Fluffy from Harry Potter and the Philosopher's Stone, a three-headed dog (inspired by Cerberus - see below) big enough to nearly fill a room. Hagrid's boarhound Fang might also qualify.
 * Also in Harry Potter, Sirius Black can transform into a Canis Major at will. (Fanon says he's a Newfoundland. The "Sirius" stories of the Paradigm of Uncertainty Fan Verse may have started this one.)
 * Orcs in The Lord of the Rings occasionally ride Wargs, which are giant wolflike beasts.
 * Also by JRR Tolkien, The Silmarillion has Huan, the huge intelligent dog (good), and Carcharoth, the huge (were)wolf (evil).
 * The Dresden Files has Mouse, who starts out small enough for Harry to keep in his pocket and quickly grows into... well, this. Harry often makes jokes about the fact that Mouse is the size of a pony, ranging from calling him a "West Highlands Dogasaurus" to, "He's a mutt. Half mastiff, half woolly mammoth."
 * The adults in Spot the Dog's world. At least, in the early books, and assuming the scale of their houses are one-to-one with those of the human world.
 * Fully grown direwolves in A Song of Ice and Fire are the size of a pony.
 * Conrad's dog Bortan in Roger Zelazny's This Immortal


 * The Ay in the Malazan Book of the Fallen. And Undead to boot.

Mythology

 * Cerberus of Greek Mythology, which also is three-headed and, depending on the story, is on fire or possesses a mane of snakes.
 * Orthros, also of Greek Mythology. The hound of Geryon, Orthros had two heads and a snake for a tail.
 * Garm of Norse Mythology, which, like Cerberus, guards the Underworld (Hel).
 * Though somewhat overlapping with Big Badass Wolf, Norse Mythology also has Fenrir, Sköll , and Hati.
 * Fenrir in particular is enormous. When he opens his mouth his lower jaw is on the ground and his upper jaw hits the sky.
 * Black Shuck, the mysterious Black Dog from Norfolk folklore, is sometimes said to be the size of a horse. Smaller than some other examples here, but still much bigger than a dog should be.

Tabletop Games

 * Magic the Gathering has Hollowborn Barghest, a very big dog. That's not dry grass it's standing in--those are trees.
 * The Space Wolves of Warhammer 40 K ride giant wolves. How giant? Well, they're big enough for a seven-foot-tall Viking Space Marine in Power Armor to ride, some reaching the size of a Rhino (whose carrying capacity is ten of the aforementioned Power Armor-wearing Space Marines).

Video Games

 * Giant wolves in the Castlevania games.
 * The Mystic Beasts in the Disgaea series are large horned canids, and seemingly a popular form of transportation, judging from some scenes in Disgaea 4 a Promise Unforgotten. In the same game, they can become even bigger in battle if another monster merges with them.
 * Orcs in War Craft games ride large wolves too.
 * Some bosses in World of Warcraft. Mostly Fenrus the Devourer. That thing is MASSIVE.
 * The Pokémon Arcanine, which is big enough to ride around on.
 * The mutated dog-boss in Fur Fighters. An unusual boss as it saw two mutated spouses instead of one. The other rode around on-top of the beast.

Web Comics

 * Wally the werewolf in Zebra Girl is much larger in canine form than an ordinary wolf.
 * The Sarghress from Drow Tales breed extra large riding wolves.
 * In Order of the Stick, Hinjo's mount Argent is a rather oversized dog/wolf. Also subverted when Belkar wishes for a riding dog.
 * In Gunnerkrigg Court, Reynardine's true form is that of a large fox, and Ysengrin is an even larger wolf. But it is Coyote himself who can become truly massive, towering over even Ysengrim.
 * Mitzi from Rhapsodies.
 * Questionable Content author Jeph Jacques is the proud owner of a Great Pyrenees, one of the huge breeds, named Shelby. Inevitably, she's appeared in his non-canon work a couple times.
 * Giant dogs serve as draft animals and guard dogs in Unsounded
 * Battlepug, follows the life of a barbarian and his oversized dog, many other oversized creatures also appear

Western Animation

 * In the episode "Ace in Hole" in Batman Beyond Terry (aka Batman) is locked in the dog fight ring with a vicious gigantic mutated mastiff.
 * The ghost dog in Danny Phantom has the ability to grow several size its normal height whenever its threatened. Lots of destruction occurs.
 * In The Fairly Odd Parents Timmy needs some enemy that would scare Catman enough to team up with his rival Crimson Chin. Cosmo has one bright moment and summons a Humongous Mecha Dog.
 * In The Legend of Korra Korra's Canine Companion is Naga, a polar-bear dog so massive that she easily serves as Korra's Horse of a Different Color, as seen here.
 * Due to the shifted scale Toby from The Great Mouse Detective qualifies. He's an ordianry pup, but all the other characters are mice.
 * Dinah from the My Little Pony episode "Pony Puppy" is in Clifford's size league.
 * Commodore Puddles (a gigantic poodle) from the Kim Possible epsidoe "Rufus vs Commodore Puddles".