终于被我在3E的FAQ里找到了
When you purchase barding for a mount, is it proficient
in the armor, or does the mount take the armor check
penalty?
Anyone wearing armor takes the appropriate armor check
penalty. Trained war mounts are proficient in light, medium,
and heavy armor, while untrained mounts are not proficient in
any kind of armor, and they take additional penalties for
untrained armor use as noted on page 80 of the Player’s
Handbook.