Screamers and Screamettes, I don't know where this mania for gas masks came from, but if you'll humor me, I promise not to indulge more than once a month.
During the joyous rush to unleash chemical hell on to the battlefields of World War I, soldiers on all sides quickly learned that humans weren't the only beasties that needed protection. Enter gas masks for horses and dogs. Gas masks for horses didn't last much longer than the First World War, when horses themselves became less and less important to modern armies (though, as late as 1948, the U.S. Army Officer Corp fielded the U.S. Olympic equstrian team). Dogs, however, remain popular instruments of war and, as you can see, there are many Cold War and later doggie gas mask designs.