Dogs have a long history of being used in warfare, going back further than the Romans and the Greeks. The vast majority of armies prior to the modern era used dogs to some extent, and they still hold important a positions in our military and police force today. The only reason they no longer have a place on our frontlines is the advent of the gun and a primary focus on ranged warfare, in which the dog like the horse has no place. However, they are still used for guarding and in K-9 units. The German Shepherd has no issue bringing down a full size human.
Historically we have the Molossus and the Alaunt which are both now extinct breeds used by the Romans. However, modern compatriots of these various breeds do exist such as the Mastiff, whose males weigh in between 150 to 250 pounds, and the Irish Wolfhound.
We have records of Irish Wolfhounds being used by the Irish to bring down Norman knights on horseback during their invasion and eventual conquest of Ireland. Their role was to catch the horse and drag the knight from the saddle to be killed.
However, it’s worth noting that dogs don’t go into battle alone. They are pack animals and they travel in teams. An armored knight wouldn’t be fighting a singular dog, he’d be fighting multiple armored dogs and possibly also their handler. These dogs when on their hind legs could almost certainly reach his throat and are more than capable of bowling him over or knocking him to the ground.
He’d be battling in melee, with the fight surging around him. So, there would be other humans
whether other knights or various soldiers who could potentially finish whatever the dogs start. Or the dogs finish whatever they start.
While versus ideas are always fun to contemplate, it’s important to remember that warfare from melee to modern is not about dueling. It’s team. Like their dogs, soldiers fight together. It’s not about the individual, but the unit.
Those who fight together, survive together.
Much as we romanticize the lone knight, CIA agent, or soldier who sticks it to the man and makes their own decisions, that’s not how warfare works. Even if you choose to go this route in your storytelling, remember that there are many participating actors taking part.
The danger of the dog is the other dogs and the man or woman standing behind them.
The good news if you want to write about war dogs is that dogs haven’t changed much and their training generally revolves around their natural instincts. So, a better understanding of medieval warfare and studying the historical usage of dogs in combat will give you a good idea of what they were used for and how to write them.
Dogs in Warfare – Wikipedia is a great place to go for a cursory look, but it is not the only stop you should make. The links at the bottom of the page are particularly helpful when it comes to establishing a wider scholarly base to work from in your research.
Dogs of War – Rome Across Europe’s article does a run down of the historical uses of various war dog types, with a focus on Rome and up to the present. If you want a focus on particular types of war dogs, you’ll need to do a scholarly dig in when it comes to historical periods.
Quartermaster War Dog Program: this page talks about the different types of dogs used in 1942, just a reminder that the usage of dogs in war extends far beyond their use on the battlefield itself. From detecting snipers, to delivering messages, and sniffing out scouts, the war dog has had many important roles throughout history that shouldn’t be overlooked.
“In France, a splendid duel was fought in 1400 between a suspected murderer and his accuser, a dog. The Chevalier Maquer killed Aubrey de Montdidier in the Forest of Bondy, near Paris, and buried the body. The only witness was Montdidier’s greyhound. The dog went back to town to a friend of his master’s and led the friend to the spot, where he whined and scratched the ground. The body was recovered and reburied, and the greyhound moved in with the friend. Shortly thereafter, it met up with Maquer and attacked him viciously; three men had to pull it off him. The dog was an otherwise gentle and amiable sort, but it kept on flying at Maquer whenever it saw him.
This was reported to the king, who decided it was definitely an accusation and arranged for the single-combat trial. The fight took place on the Ile de France in Paris, Maquer with a lance, the greyhound with its natural weapons. The dog sprang on the man with amazing ferocity and clamped its teeth around his throat and couldn’t be shaken off. Maquer screamed that he’d confess if they’d pull off the dog.
This, in contemporary eyes, proved the justice of combat trials pretty conclusively, and Maquer was hanged and strangled on the gibbet and Montfaucon.” -Gentleman’s Blood: A History of Dueling
There’s also this.