In addition to humans, all of the animals used in WWI, including horses, pigeons, dogs, and mules, had to be outfitted with at least some degree of protection from the gas attacks going on all around them.
All animals suffered from inhalation injuries from the lung irritants, but the threat to the eyes and skin of war animals was less than in humans. Horse and mule eyes were noted to show remarkable healing power, even after vesicant (i.e. mustard gas - blister gases) contamination.
Image from US Army Chemical Corps Museum, taken by unknown Entente powers soldier in France.