buying a trained animal and then making it my companion

[KenderKin]

So can I buy a guard dog (already trained) and then later make it my animal companion?

Seems like it would gain a new trick but would already be combat trained or am I missing something important?

[asthyril]

yes it would already have the 4 tricks necessary to be a guard dog per the handle animal skill, leaving 2 open tricks it can learn (one of which you should probably use to teach it the attack skill again, so it will attack undead and other weird things it normally wouldn't attack)

when it became an animal companion it would get 1 bonus trick, making the total it could learn 7, increasing as it increases in level.

note you may want to talk to your GM about using tricks to teach your animal companion how to use spells cast on it, such as spider climb or fly, using the air walk spell as a guide for how to do it.