I could excuse that considering history in his time will have been more difficult to research. Where would the poor man have found a picture of Roman armor? To see a horse on the other way he probably just needed to go to the street.

Most people at the time knew that in the past and elsewhere, clothes and armor were different. If nothing else, even the uneducated had heard stories from the Bible that detail the kinds of clothes people wore, and anyone educated enough to be painting for a living or illustrating manuscripts would have studied older artwork including some from the Romans. The difference is that they considered historical accuracy much less relevant than relating to their audience, so they presented subjects in the clothes that would immediately and obviously portray their station. The artist of the Morgan Leaf from the Winchester Bible would have seen changes in arms and armor over his lifetime, and he would have known that the people in these scenes from the life of David would not have looked like 11th-century natives as portrayed:

Once they understood the difference, they would have considered it odd that we don’t portray 11th-century knights in Kevlar kneeling before a king in a tailored suit, though they would also have deplored the disgrace caused by the plainness in which our kings dress.

I know, gift horses and mouths and all that, but I wouldn’t adopt the poor creature even as a rescue. Put it out of its misery. Wasn’t there a special horse hating community? I’m sure the animal belongs there!