Wasn't armor obsolete by the 18th century? Maybe used again in a limited way in the 20th century.
How could a horse function covered with armor?
Decorative armor was a HUGE business in the 18th and 19th centuries. Everyone wanted some armor to display at home or in a museum or business, but there wasn't enough authentic stuff to go around. So a new generation of armorers made attempts to restore or complete actual medieval suits, or made deliberate fakes, or just made reproductions "in the style of" something older and meant to be used to decorate your parlor. Modern collectors (and sometimes museums!) often have a hard time telling the real stuff from the later imitations, but it can be done.
Horses were definitely armored! Even in Roman times, heavy cavalry horses could be covered in scale armor. By the middle ages, padding or mail was used, and plate armor from the 14th century onwards. Not *all* horses, by any means, but quite a few. Worked fine, and kept your expensive horse alive.
Matthew