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 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 Sajin Komamura is the largest Gotei captain and a wolf. He has nothing on his unnamed ancestor, who turns out to be even bigger - to put it on perspective, the Leader is at least five times bigger than Koma himself.
In InuYasha, 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
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 considerably bigger compared to them.
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!
John Constantine of Hellblazer once had to put down a ghost-possessed bulldog the size of a rhino.
Lockjaw from The Inhumans, though for a while he was considered an Inhuman mutated into his current form, as per John Byrne. This later got retconned into Gorgon and Karnak playing a cruel joke on the Thing.
Maxipes Fik: Maxipes (dog=pes) Fík, the dog hero from Czech animated shorts for children. When a girl named Ája (pronounced Ah-ya) got him as a puppy, he was normal-sized, but he grew to enormous size. He grew even bigger when he drank a barrel of beer, and Ája's father had to build a garage-like dog-house for Fík.
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.
"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.
Fenrir, the son of Loki and a giantess, who bit off the hand of the god Tyr; fated to kill Odin. Fenrir in particular is enormous. When he opens his mouth his lower jaw is on the ground and his upper jaw hits the sky.
Sköll who is fated to consume Sól, aka the sun.
Hati who is fated to consume Máni, aka the moon.
If you go back far enough the Valkyries rode wolves the size of war horses,
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.
Werewolves in Werewolf: The Apocalypse and Werewolf: The Forsaken can take on five forms, including a 800 pound bipedal wolf-headed war-beast and a dire wolf that is only marginally smaller and even more nimble to make up for the lack of thumbs.
Various forms of giant wolves (and, sometimes, other canids and vulpines) have shown up throughout Dungeons & Dragons and Pathfinder. The iconic three, which seem to show up in every setting, are Dire Wolves (regular wolves, but bigger and stronger), Wargs/Worgs (Dire Wolves with human-like intelligence and evil alignments), and Winter Wolves (Wargs that have a freezing cold bite attack andBreath Weapon).
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.
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.
Dark Souls has the Great Grey Wolf Sif. A giant wolf that wields a giant sword.
Old Red in Professor Layton and the Azran Legacy is, as his name indicates, a huge old red-furred wolf. He's so threatening-looking that he constantly scares the town of Torrido. Which is actually completely unintended, as he merely has the Face of a Thug: he just wants to see his old friend again.
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.
All Night Laundry has one. At the beginning he's big, but not impossibly big. He's been bigger in each of his appearances since due to damage to Bina's Timeline.
Cry Havoc's werewolves become this with a full moon. They stand about 6 feet tall at the shoulder, and are capable of standing on two legs, raising their height to nearly 12 feet. Needless to say, they also weigh more than most cars.
Quest of Camelittle has Black Fang, a creatively-named wolf puppy who can grow bigger at his own will.
In Web Serial NovelWorm, one character, Bitch, has the ability to make dogs temporarily grow into giant, spiky monsterous forms. When she gets a wolf cub, it definitly fits this trope when transformed, particually as it transforms better than the dogs do (it's less deformed by the transformation), possibly because wolves are just better.