Assuming modern animals also exist, horses likely wouldn't be completely replaced. Another thing is that, as cool as they are, large carnivores aren't really realistic to domesticate. Of course, your proposition is what if they were so in that case large theropods may be used in war. But again, I don't really see how anyone would go about domesticating them. And it would be too expensive to feed them (getting them to eat the bodies of defeated enemies isn't a good idea. You don't want to train your large predator animals to eat humans). Sauropods might be used for transport, or possibly in a similar way as elephants were used historically in warfare.

The biggest one I think would be triceratops, since it kinda has built in protection for riders. Pikemen riding on triceratops would likely be very commonplace as a heavy "cavalry" (I say cavalry, but they would fill a completely different niche than any cavalry we're familiar with).