Looking at what we have for dogs today really dose not give a good indication of the D&D riding dog.
The first thing is, our world never bred canines for combat riding, unlike the horse, which got centuries of it. In our modern world, a dog’s Constitution gets dump-stated to maintain the Breed Standard [Cha?]. On the most part, even Sentry / Guard / Attack dogs are bred Breed first, purpose second.
In D&D worlds, breeders of wardogs will be choosing purpose first, mixing bloodlines intentionally and often to make a better mount. Someone selling inbred war dogs with bad hips will find their reputation destroyed, if they are not run through by someone’s next of kin. The nobles can have their inbred pets, those going to the battlefield will choose bad coat colors over bad health.
Want an Idea what a D&D war mount dog would be like? Look at the breed mixing Karl Friedrich Louis Dobermann did to breed a dog to guard himself while tax collecting. He mixed so many dog breeds together no one can say for certain which ones made the breed. A breed which is very good at what it does.
Now picture the largest breeds of dogs and start mixing them in that manner for strength, stamina and the ability to bear a rider. And if the campaign lets that riding dog get to large size, you may wind up with a mount whose loyalty to the rider makes the “Faithful Steed” seem like a traitorous coward.
I like picturing Newfoundland like riding dogs for riders who may need to take watery paths.
Now picture if they were
trying to breed mount size