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 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. 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!
Maxidog Fik: Maxi-dog 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.
Cerberus of Greek Mythology, which also is three-headed and, depending on the story, is on fire or possesses a mane of snakes.
Orthros 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).
Norse Mythology has 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.
Norse Mythology has Sköll who is fated to consume Sól, aka the sun.
Hati of Norse Mythology (Fated to consume Máni, aka the moon).
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.