As I was prepping for a halloween oneshot, one of my players started to ask questions regarding the effects of polymorph spells (specifically beast shape II granted through a skinwalker feat for werebat-kin). One of the questions they had related to their plan of assuming the form of a dire bat & have another person help put barding on them.
As I tried to look things up for them, I noticed that barding has the line that, "Flying mounts can't fly in medium or heavy barding." However, the player obviously isn't a mount - so is there any other rule I'm missing about how barding interacts with a polymorphed player who is flying? (said player wants full-plate barding).
Personally, it'd seem a bit weird to me if flying creatures can fly just fine in heavy armor - until the moment they become a mount and are suddenly unable to fly at all. But I haven't managed to find anything which would support this...
That line means if you have a flying creature that is acting as a mount, it cannot fly while carrying a rider if it has medium or heavy armor even if it has the strength to do so.
I would use the Armor for Unusual Creatures rules for pricing his armor. For a large non-humanoid armor is x4 cost and x2 weight.
For fullplate that would be 6,000GP and would weigh 100lbs. Beast shape 2 gives him up to a 60 foot fly speed, but I'm guessing he is shaping into a Dire Bat which has 40 feet (good). His fly speed in armor would be 30 feet.
Creatures can fly just fine in heavy armor, until you put a rider on them, then they cannot. They aren't a "mount" until someone is riding them.