My brother is telling me I can't have dwarves in my book breed large dogs (similar to breeds like the Tibetan Mastiff or Irish Wolfhound) to ride into battle because it's "scientifically inaccurate." He says that it wouldn't work because animals big enough and aggressive enough for that can't be domesticated for breeding. They can only be tamed. Does this mean I should come up with something else? Or could I get away with it if I was consistent with it? And aware of why it wouldn't work irl?
That doesn’t sound real.
I mean, there is a distinct difference between taming and domestication, but just because something isn’t domesticated doesn’t mean it can’t learn different behaviors from its wild counterparts. In fact, that’s exactly what tamed house cats do (house cats are not domesticated). Even horses, though domesticated, have not been so genetically altered that they could not, given the right circumstances, survive in the wild. (Here are some more answers to the question of the difference between the two.)
Also, it’s your story and your world. Perhaps you use canine-like creatures without explicitly making them dogs and then you can do whatever the hell you want because it’s a species you created.
You could breed anything you wanted, you might just have to exert a stricter level of control over their day-to-day lives to prevent unwanted mating and uncouth behaviors.
Bottom line: it’s fiction. If you keep your depiction of this species’ tameness or domestication informed, then you’ll be fine.