A Riding Dog Question

[Eldred]

The SRD of the Riding Dog states that:

If trained for war, these animals can make trip attacks just as wolves do (see the Wolf entry). A riding dog can fight while carrying a rider, but the rider cannot also attack unless he or she succeeds on a Ride check.

So, what consitutes the Riding Dog as being trained for war? I'm planning to play a Druid (likely a Gnome or Halfling) to fight mounted on a Riding Dog animal companion, but does that mean it needs to be trained for war beforehand? If so, what do I need to do that?

[Attilargh]

A Handle Animal check. However, like a Warhorse, a Riding Dog has already been trained to bear a rider into battle.

[cupkeyk]

A creature gets tricks per point of intelligence. You can decide which tricks your riding dog knows when you purchase it. A fighting mount, that will occupy three tricks: accept rider, fight, fight unnatural creatures. You will also want come (to make it follow you in places it will otherwise run from) and defend.

[Attilargh]

...Or you could simply use the "Train an Animal for a Purpose" feature of the Handle Animal:

Train an Animal for a Purpose

Rather than teaching an animal individual tricks, you can simply train it for a general purpose. Essentially, an animal’s purpose represents a preselected set of known tricks that fit into a common scheme, such as guarding or heavy labor. The animal must meet all the normal prerequisites for all tricks included in the training package. If the package includes more than three tricks, the animal must have an Intelligence score of 2.

...

Combat Riding (DC 20): An animal trained to bear a rider into combat knows the tricks attack, come, defend, down, guard, and heel.