List of Fictional Dogs.

A sampling:

Buster, Scottish Terrier belonging to Frederick Algernon Trotteville ("Fatty") in Enid Blyton's "Five Find-Outers" series of children's mystery novels
Cafall, a dog belonging to King Arthur (occasionally spelled "Cabal")
Cerberus from Greek Mythology, the three-headed hellhound of Hades.
Ceril, the hellhound from Eva Ibbotson's The Haunting of Hiram.
Chief from The Fox and the Hound
Copper from The Fox and the Hound
Cujo the St. Bernard in the novel by Stephen King (later a movie)
Dingo, in Dick Sand, A Captain at Fifteen by Jules Verne
Dingus, in Dingus Dreaming and The Canine Condition by Alex A. Vardamis
Dog, the hellhound from Good Omens by Terry Pratchett and Neil Gaiman
Dog that did nothing in the night-time, unnamed, this "curious incident" providing a clue in the Sherlock Holmes story The Adventure of Silver Blaze by Sir Arthur Conan Doyle.
Duchess, the dog who thought she swallowed a patty pan, in Beatrix Potter's The Tale of the Pie and the Patty-Pan
Effdee, the protagonist on the fantasy webcomic Anema: Age of the Robots
Einstein, the golden retriever in Dean Koontz's book Watchers
Fang, Grip and Wolf. Farmer Maggot's guard dogs in J. R. R. Tolkien's The Lord of the Rings
Fang, Hagrid's dog, a boar-hound (a Mastiff) in the Harry Potter books: (in the movies, Fang is a Neapolitan Mastiff)
Fidele in Gogol's Diary of a Madman
Fluke, the title character in Fluke by James Herbert
Fluffy, the three-headed dog (similar to Cerberus) in Harry Potter and the Philosopher's Stone
Fly, the collie that befriends Babe the piglet in Dick King-Smith's The Sheep-Pig, the basis for the film Babe
Hush Puppy, one of Shari Lewis's puppets
Happy Walter Higgenbottom and many of dogs from The Mighty B on Nickelodeon
Jasper, Brian Griffin's effeminate gay cousin, Family Guy
Jock, the Scottish Terrier from Disney's Lady and the Tramp and Lady and the Tramp II: Scamp's Adventure
Kipper, a young dog who always has adventures waiting for him
K-9 a Looney Tunes character
Krypto, the superpowered pet dog of Superman, in DC comics.
Laddie, a dog which was once owned by Bart Simpson in an episode of The Simpsons
Lady, the Cocker Spaniel from Disney's Lady and the Tramp and Lady and the Tramp II: Scamp's Adventure
Ladybird, the Hills' pet bloodhound, in King of the Hill
Lafayette, the Basset Hound from Disney's The Aristocats
Little Brother, Mulan's dog in Disney's Mulan
Loyal Heart Dog - a Care Bears cousin
Lucy - Alvin and the Chipmunks
Lucky, from the 101 Dalmatians series
Marc Antony, Looney Tunes character
Max, the Sheep Dog from The Little Mermaid
Max, the Grinch's companion in Dr. Seuss' How the Grinch Stole Christmas!
Max Goof, son of Goofy
Maxipes Fik
Mighty Manfred the Wonder Dog, Tom Terrific's companion on Captain Kangaroo
Mumbly, from The Mumbly Cartoon Show and Laff-A-Lympics in 1977
Muttley, Dick Dastardly's sidekick in Wacky Races with the craziest laugh, Dastardly and Muttley in their Flying Machines and Yogi's Treasure Hunt
Mr. Peabody, genius dog and history expert in The Rocky and Bullwinkle Show
Mr. Pendleton from What ever Happened to Robot Jones
Ms. Lion from Spider-Man and his Amazing Friends
Napoleon, the Bloodhound from Aristocats
Precious, the white Toy poodle beloved of 'Buffalo Bill' in The Silence of the Lambs
Quark, from Honey, I Shrunk the Kids
Rexxx/Dewy, from Firehouse Dog
Riff Raff, from Underdog
Rin Tin Tin, a German Shepherd found in a trench by an American soldier, has appeared in films since 1922. All subsequent dogs in the part have been descendants of the original. Also on television.
Reno, from the movie Top Dog starring Chuck Norris
Rocky, Captain Felix Maxwell's dog in Mannequin
Russell, from Celtic Pride
Samantha, from the film I Am Legend starring Will Smith
Sasha, from The Last Shot
Shadow, from the film Homeward Bound: The Incredible Journey, voiced by Don Ameche.
The Shaggy Dog
Shithead, from the movie The Jerk starring Steve Martin

THE LIST GOES ON! (Complete list of nonfictional dogs still under construction.)

Thx to Leah.