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 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.
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.