Druid has an animal companion and wants to armor it up. Assuming that he doesn't spend a feat to get armor prof. what stops the animal from wearing barding?
EDIT
Under the entry for horse there is nothing stating that they have feats relating to armor so I assume that wearing barding requires no proficiency.
I have always thought that if you train a animal for combat you train it to wear armor as well. I don't think that it was that overpowering. Now for animals that will be functioning in combat other than as a mount, I don't consider requiring a feat to be a bad requirement. The druids animal companion is just not designed with armor wearing in mind, and if you add that it should require something to do. Personally I like the idea of a druid riding into combat with a armored dinosaur so I would allow it.