It then has combat training, plus two other tricks. The Handle Animal skill explains (end of page 97): "Essentially, an animal's purpose represents a preselected set of known tricks .. [p. 98] The new general purpose and tricks completely replace the animal's previous purpose and any tricks it once knew". Note that the latter sentence specifically relates to training a standard INT 2 horse for combat, taking 6 tricks, that previously had the purpose of riding, which takes 3 tricks.

Got it, thank you!