During World War I soldiers relied on horses for transportation on the battlefield. The introduction of chemical warfare at this time meant horses had to be just as protected as their human counterparts so they were fitted with gas masks over their muzzles to prevent them from inhaling poison gases such as chlorine and phosgene.
Link - Via Archie McPhee
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The other compound is Mercury(II) thiocyanate (Hg(SCN)2) AKA "Pharoah's Serpent" It used to be used in fireworks because of its black-snake effect. However, it isn't readily available anymore because it is toxic.