Unless someone can throw something more convincing "BTB" at me, I usually say that all animals are non-proficient with any armor. Thus, you want to put them in padded, leather, masterwork studded leather, or mithril chain shirt barding in most cases.
(a) More convincing than a direct quote stating it? What do you want, exactly?
(b) If they're not proficient with any armor, why would those particular options be applicable "in most cases"? They're still non-proficient.
A paladin's mount is a warhorse or a warpony (although other variants are allowed). I would consider it logical to assume that alternate mount types are war-trained, and (thus) proficient with armor. But I suppose some DMs might argue that the paladin will first need to arrange for their special mount to be war-trained if the mount's type doesn't specify war-training already.
