Why bother with the proficiency?
Leather Armor has an ACP of 0, and Hide Armor isn't worth it, really.
Just get some Leather Armor Barding and your Animal Companion has +2 AC, and it's cheap!
I mean, although there's no rules against it, I wouldn't allow a Druid Animal Companion to use restricted armor...although a Ranger one might be ok.
Still, Ranger Animal Companions probably shouldn't be fighting too much...
*edit* They most CERTAINLY do get feats. Check your friendly Animal Companion Sidebar.