Shin Okada said:
Animals trained for war does not automatically get armor proficiency. War horses are not proficient in armor and thus suffer attack penalty when they make hooves/bite attacks.
Well, I think in the RAW, warhorses are definitely proficient with some kind of armor, though what specific armors is not clear. Otherwise, the line under the Animal Type entry in the SRD would make no sense. Saying "proficient with no armor unless trained for war" can only mean (it seems to me,) that if trained for war, they're proficient in some kind of armor.
Still, the main crux of my query was regarding a magical beast mount, since the Magical Beast entry does not contain the same caveat regarding being trained for war. Either they simply forgot that there are a couple magical beasts that can be specifically trained to serve as mounts (hippogriff, griffon, etc.) or they intentionally did not want such mounts to be capable of wearing armor without an additional price, such as a feat.
Now, I'm my particular case the mount does have an extra feat (it's a paladin's griffon mount) to spend if the rules require it, though I'd earmarked that for another feat, and would be disappointed to have to spend it on armor proficiency.
FireLance said:
you can just sidestep the issue by getting barding that has no armor check penalty, e.g. leather, masterwork studded leather, or mithral chain shirt barding.
Good point. As the mount I have in mind must maintain a light load (to be able to carry my paladin aloft,) I'd been eyeing chain shirt armor at the heaviest. But it may be that I'll have to get studded leather at first, then save up enough gold to commission mithril chain shirt barding later... An added cost, but at least it wouldn't require me to spend one of the mount's feats to be protected when riding/flying into combat.