I remember reading in a first-party book that the ability to wear barding could be taught as an animal trick. The internet in general seems to take this as a given. But I just spent quite a while looking for this rule and couldn't find it.
- It doesn't appear to be with the rules for barding.
- It doesn't appear to be in Handle Animal.
- It doesn't appear to be in the equipment-section rules for mounts.
- I couldn't find it in the combat chapter.
I know this is a fairly trivial question, but it's also one which I haven't been able to answer.
Alternately, if my premise is wrong, what are the rules for being able to wear barding?