There are countless depictions of Cerberus with various number of heads or additional parts such as snake heads or tails. Some even say that Cerberus did not refer to a dog or hound at all, but rather a snake.
The most iconic modern interpretation that comes to mind (for me and through ignoring Shin Megami Tensei series) is a breed that resembles a rottweiler, but what breeds would also work?
Cerberus was the son of Echidna, a half-woman half-snake, and Typhon, a huge serpentine horror. That's a hell of a pedigree for a dog.
More seriously, I imagine Cerberus had to be at least loosely based on the breeds already known by the ancient Greeks, and those were the Laconian and the Molossian hounds.
The Molossian breed was more robust and fierce, and was trained in combat from ancient times, so it is a more likely candidate to form a basis for the Cerberus myth.
There's more info about ancient Greek dogs in this page: Dogs in ancient Greece.
Also found a book about Plutarch where the link between Cerberus and the Molossian hound is quite explicit:
Theseus and Perithous stole Helen, who went by lot to Theseus. Then the two set out to northwest Greece to find a wife for Perithous. There they encountered the Molossian king Aidoneus ("Hades"), a possessive father with a beautiful daughter named Kore ("Maiden," the euphemistic alternate name for Persephone in myth and cult) and a wife named Persephone. Loath to see Kore married, Aidoneus was accostumed to force her suitors to wrestle with his hound Cerberus - Molossian hounds were proverbially big and fierce - and that was the end of Perithous.
Plutarch, by Robert Lamberton.
Cerberus was known as the hound of Hades. This three headed dog was the offspring of Echidna and Typhon. He had three heads, a serpent tail, and snakes coming out of some of his body parts.
The Cerberus was three headed but not all the time when written or talked about. He was related to many species that had many heads. Cerberus father had many snake heads who was called Typhon. Echidna, who was his mother, was half snake. The Cerberus was the brother of other multi-headed monsters that include Lernaean Hydra, Orthrus, and a two headed dog that guarded the cattle of Geryon.
The other one was Chimera who had the three heads of a lion, goat, and a snake. Cerberus was the only rare multi-headed animal talked about that had a few too many heads. In Hesiods Theogonay the Cerberus had fifty heads. Pindar gave the Cerberus one hundred heads. Many writers later on would give the Cerberus three heads.
There is one story by the poet Horace where the Cerberus has one dog head and a hundred snake heads. In Appollodorus the Cerberus has three heads with a bunch of snakes. In the story of the poet John Tzetzes the Cerberus had fifty heads of which three were of a dog and the rest of many beasts. In art from back then the Cerberus is often depicted with having only two heads or just one head. In 590 bc to 580 bc the Cerberus in the Corinthian cup of Argos had only one head.
When the Cerberus started showing to have three heads was in the mid sixth century BC on the Laconian cup. The Cerberus was usually depicted as having a snake or dog body with the heads and tails being of that sort. There were other cases were the Cerberus had three bodies that went with the three heads and had three tongues as well. In a Heracles story the Cerberus would vomit out bile which the poisonous aconite plant grew from. The vomiting started when the Cerberus was being taken out the underworld by Heracles.
On some accounts the Cerberus would have fire coming out his mouth or a foam of poison that would come out. They never would say what kind of breed of canine the Cerberus was, but if they can't refer to the Cerberus as a Hell Hound then it was a Hound.
The Cerberus was made up of canine, lion, goat, and snake in many stories.
In Henrique C. Agrippa's "occult philosophy" book 2, in his commentary in chapter 18 on Cerberus he focuses on the elements air, water and earth (about the 3 heads) symbology, nothing of the breed is mentioned, because for the symbology it is relevant the aspects that refer to this elements. I suppose the breed is a very concrete detail, so they bring that kind of representation to the surface in animes, movies, etc. to be used, but it is hard to define this for sure!
