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.
Akamaru grows quite huge after the Shippuden timeskip. And don't even think about Pain's Giant Multi-Headed Dog, one of the most perfectly terrifying creatures incarnate.
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 Komamura is the largest captain. He's also a Big Badass Wolf even though characters in-universe think he's a dog.
In Inu Yasha, Sesshomaru, his father and his mother all have true forms that are this. In parts of the manga not yet officially published in English, there is a giant mountain dog and a giant fox kami
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!
The one-and-only Fluffy from Harry Potter And The Philosophers 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 UncertaintyFan Verse may have started this one.)
Orcs in The Lord of the Rings occasionally ride Wargs, which are giant wolflike beasts.
Also by J. R. R. 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.
Conrad's dog Bortan in Roger Zelazny's This Immortal
Was there ever another dog the size of a small horse, with armor plates on his sides, and jaws like a trap for bears?
The Ay in the Malazan Book Of The Fallen—and Undead to boot, as they've been extinct for about 320,000 years, when the T'lan Imass decided to include them in the Ritual of Tellann, which made the Imass undead.
(The son of Loki and a giantess, who bit off the hand of the god Tyr; fated to kill Odin)
, Sköll*
(Fated to consume Sól, aka the sun)
, and Hati*
(Fated to consume Máni, aka the moon)
.
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.
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. In the same game, they can become even bigger in battle if another monster merges with them.
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 becometruly massive, towering over even Ysengrim.
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 coupletimes.
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
