I stumbled across a compelling photo (below) and some great links at we make money not art, exploring the history of gas masks for animals in wartime.
There’s a great essay by Major Robert D. Walk outlining the development of protection for animals here:
With the introduction of chemical warfare in World War I, soldiers had to protect themselves and their animals in order to survive and fight. Horses, mules, dogs, and pigeons all required protection from the effects of chemical warfare.
Each animal presented different challenges in protection. Horses breathe through their noses, not through their mouths, so the respirator must only cover their nostrils. Horses’ eyes are not affected by lacrimators (tear agents), so eye protection initially was not required. More
Don’t miss this either; lots of fantastic old photos.