It was my understanding that a "Riding Dog" was, by default, trained to be a war mount.
Edit: I was referring to this:
Combat Riding (DC 20): An animal trained to bear a rider into combat knows the tricks attack, come, defend, down, guard, and heel. Training an animal for combat riding takes six weeks. You may also ďupgradeĒ an animal trained for riding to one trained for combat riding by spending three weeks and making a successful DC 20 Handle Animal check. The new general purpose and tricks completely replace the animalís previous purpose and any tricks it once knew. Warhorses and riding dogs are already trained to bear riders into combat, and they donít require any additional training for this purpose.